feat(display): Add new peripheral status/display

* Add new API/status to track state of the
  peripheral connection to the central.
* Add new peripheral status widget for displaying
  the current status of the connection to
  the central.

app/include/zmk/display/widgets/peripheral_status.h

@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) 2022 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+#include <lvgl.h>
+#include <kernel.h>
+
+struct zmk_widget_peripheral_status {
+    sys_snode_t node;
+    lv_obj_t *obj;
+};
+
+int zmk_widget_peripheral_status_init(struct zmk_widget_peripheral_status *widget,
+                                      lv_obj_t *parent);
+lv_obj_t *zmk_widget_peripheral_status_obj(struct zmk_widget_peripheral_status *widget);

app/include/zmk/events/split_peripheral_status_changed.h

@@ -0,0 +1,16 @@
+/*
+ * Copyright (c) 2022 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+#include <zephyr.h>
+#include <zmk/event_manager.h>
+
+struct zmk_split_peripheral_status_changed {
+    bool connected;
+};
+
+ZMK_EVENT_DECLARE(zmk_split_peripheral_status_changed);

app/include/zmk/split/bluetooth/peripheral.h

@@ -0,0 +1,9 @@
+/*
+ * Copyright (c) 2022 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+bool zmk_split_bt_peripheral_is_connected(void);