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/main.c

@@ -0,0 +1,56 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <init.h>
+#include <device.h>
+#include <devicetree.h>
+
+#include <logging/log.h>
+LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
+
+#include <drivers/display.h>
+#include <lvgl.h>
+
+#include <zmk/display/status_screen.h>
+
+#define ZMK_DISPLAY_NAME CONFIG_LVGL_DISPLAY_DEV_NAME
+
+static struct device *display;
+
+static lv_obj_t *screen;
+
+__attribute__((weak)) lv_obj_t *zmk_display_status_screen() { return NULL; }
+
+int zmk_display_init() {
+    LOG_DBG("");
+
+    display = device_get_binding(ZMK_DISPLAY_NAME);
+    if (display == NULL) {
+        LOG_ERR("Failed to find display device");
+        return -EINVAL;
+    }
+
+    screen = zmk_display_status_screen();
+
+    if (screen == NULL) {
+        LOG_ERR("No status screen provided");
+        return 0;
+    }
+
+    lv_scr_load(screen);
+
+    lv_task_handler();
+    display_blanking_off(display);
+
+    LOG_DBG("");
+    return 0;
+}
+
+void zmk_display_task_handler() {
+    lv_tick_inc(10);
+    lv_task_handler();
+    k_sleep(K_MSEC(10));
+}