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/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);