feat(display): Initial widget/status screen work.

* Battery and output status widgets
* Built in status screen combining them.
* Ability to define a custom status screen factory
  function.

app/src/display/widgets/CMakeLists.txt

@@ -0,0 +1,3 @@
+
+target_sources_ifdef(CONFIG_ZMK_WIDGET_BATTERY_STATUS app PRIVATE battery_status.c)
+target_sources_ifdef(CONFIG_ZMK_WIDGET_OUTPUT_STATUS app PRIVATE output_status.c)

app/src/display/widgets/Kconfig

@@ -0,0 +1,18 @@
+
+menu "ZMK Display Widgets"
+
+config ZMK_WIDGET_BATTERY_STATUS
+    bool "Widget for battery charge information, using small icons"
+    depends on BT
+    default y if BT
+    select LVGL_OBJ_LABEL
+    select LVGL_BUILD_IN_FONT_ROBOTO_16
+
+config ZMK_WIDGET_OUTPUT_STATUS
+    bool "Widget for keyboard output status icons"
+    depends on BT
+    default y if BT
+    select LVGL_OBJ_LABEL
+    select LVGL_BUILD_IN_FONT_ROBOTO_16
+    
+endmenu

app/src/display/widgets/battery_status.c

@@ -0,0 +1,85 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <bluetooth/services/bas.h>
+
+#include <logging/log.h>
+LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
+
+#include <zmk/display/widgets/battery_status.h>
+#include <zmk/usb.h>
+#include <zmk/events/usb-conn-state-changed.h>
+#include <zmk/event-manager.h>
+#include <zmk/events/battery-state-changed.h>
+
+static sys_slist_t widgets = SYS_SLIST_STATIC_INIT(&widgets);
+static lv_style_t label_style;
+
+void battery_status_init() {
+    if (label_style.text.font != NULL) {
+        return;
+    }
+
+    lv_style_copy(&label_style, &lv_style_plain);
+    label_style.text.color = LV_COLOR_BLACK;
+    label_style.text.font = &lv_font_roboto_16;
+    label_style.text.letter_space = 1;
+    label_style.text.line_space = 1;
+}
+
+void set_battery_symbol(lv_obj_t *label) {
+    char text[2] = "  ";
+    u8_t level = bt_gatt_bas_get_battery_level();
+
+#if IS_ENABLED(CONFIG_USB)
+    if (zmk_usb_is_powered()) {
+        strcpy(text, LV_SYMBOL_CHARGE);
+    }
+#endif /* IS_ENABLED(CONFIG_USB) */
+
+    if (level > 95) {
+        strcat(text, LV_SYMBOL_BATTERY_FULL);
+    } else if (level > 65) {
+        strcat(text, LV_SYMBOL_BATTERY_3);
+    } else if (level > 35) {
+        strcat(text, LV_SYMBOL_BATTERY_2);
+    } else if (level > 5) {
+        strcat(text, LV_SYMBOL_BATTERY_1);
+    } else {
+        strcat(text, LV_SYMBOL_BATTERY_EMPTY);
+    }
+    lv_label_set_text(label, text);
+}
+
+int zmk_widget_battery_status_init(struct zmk_widget_battery_status *widget, lv_obj_t *parent) {
+    battery_status_init();
+    widget->obj = lv_label_create(parent, NULL);
+    lv_label_set_style(widget->obj, LV_LABEL_STYLE_MAIN, &label_style);
+
+    lv_obj_set_size(widget->obj, 40, 15);
+    set_battery_symbol(widget->obj);
+
+    sys_slist_append(&widgets, &widget->node);
+
+    return 0;
+}
+
+lv_obj_t *zmk_widget_battery_status_obj(struct zmk_widget_battery_status *widget) {
+    LOG_DBG("Label: %p", widget->obj);
+    return widget->obj;
+}
+
+int battery_status_listener(const struct zmk_event_header *eh) {
+    struct zmk_widget_battery_status *widget;
+    SYS_SLIST_FOR_EACH_CONTAINER(&widgets, widget, node) { set_battery_symbol(widget->obj); }
+    return 0;
+}
+
+ZMK_LISTENER(widget_battery_status, battery_status_listener)
+ZMK_SUBSCRIPTION(widget_battery_status, battery_state_changed);
+#if IS_ENABLED(CONFIG_USB)
+ZMK_SUBSCRIPTION(widget_battery_status, usb_conn_state_changed);
+#endif /* IS_ENABLED(CONFIG_USB) */

app/src/display/widgets/output_status.c

@@ -0,0 +1,95 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <bluetooth/services/bas.h>
+
+#include <logging/log.h>
+LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
+
+#include <zmk/display/widgets/output_status.h>
+#include <zmk/event-manager.h>
+#include <zmk/events/usb-conn-state-changed.h>
+#include <zmk/events/ble-active-profile-changed.h>
+#include <zmk/usb.h>
+#include <zmk/ble.h>
+#include <zmk/endpoints.h>
+
+static sys_slist_t widgets = SYS_SLIST_STATIC_INIT(&widgets);
+static lv_style_t label_style;
+
+void output_status_init() {
+    if (label_style.text.font != NULL) {
+        return;
+    }
+
+    lv_style_copy(&label_style, &lv_style_plain);
+    label_style.text.color = LV_COLOR_BLACK;
+    label_style.text.font = &lv_font_roboto_16;
+    label_style.text.letter_space = 1;
+    label_style.text.line_space = 1;
+}
+
+void set_status_symbol(lv_obj_t *label) {
+    enum zmk_endpoint selected_endpoint = zmk_endpoints_selected();
+    bool active_profile_connected = zmk_ble_active_profile_is_connected();
+    bool active_profie_bonded = !zmk_ble_active_profile_is_open();
+    u8_t active_profile_index = zmk_ble_active_profile_index();
+    char text[6] = {};
+
+    switch (selected_endpoint) {
+    case ZMK_ENDPOINT_USB:
+        LOG_DBG("USB, BOY!");
+        strcat(text, LV_SYMBOL_USB "   ");
+        break;
+    case ZMK_ENDPOINT_BLE:
+        if (active_profie_bonded) {
+            if (active_profile_connected) {
+                LOG_DBG("Bonded & connected!");
+                sprintf(text, LV_SYMBOL_WIFI "%i " LV_SYMBOL_OK, active_profile_index);
+            } else {
+                LOG_DBG("Bonded but not connected!");
+                sprintf(text, LV_SYMBOL_WIFI "%i " LV_SYMBOL_CLOSE, active_profile_index);
+            }
+        } else {
+            LOG_DBG("NOT Bonded!");
+            sprintf(text, LV_SYMBOL_WIFI "%i " LV_SYMBOL_SETTINGS, active_profile_index);
+        }
+        break;
+    }
+
+    lv_label_set_text(label, text);
+}
+
+int zmk_widget_output_status_init(struct zmk_widget_output_status *widget, lv_obj_t *parent) {
+    output_status_init();
+    widget->obj = lv_label_create(parent, NULL);
+    lv_label_set_style(widget->obj, LV_LABEL_STYLE_MAIN, &label_style);
+
+    lv_obj_set_size(widget->obj, 40, 15);
+    set_status_symbol(widget->obj);
+
+    sys_slist_append(&widgets, &widget->node);
+
+    return 0;
+}
+
+lv_obj_t *zmk_widget_output_status_obj(struct zmk_widget_output_status *widget) {
+    return widget->obj;
+}
+
+int output_status_listener(const struct zmk_event_header *eh) {
+    struct zmk_widget_output_status *widget;
+    SYS_SLIST_FOR_EACH_CONTAINER(&widgets, widget, node) { set_status_symbol(widget->obj); }
+    return 0;
+}
+
+ZMK_LISTENER(widget_output_status, output_status_listener)
+#if defined(CONFIG_USB)
+ZMK_SUBSCRIPTION(widget_output_status, usb_conn_state_changed);
+#endif
+#if defined(CONFIG_ZMK_BLE)
+ZMK_SUBSCRIPTION(widget_output_status, ble_active_profile_changed);
+#endif