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/include/zmk/display/status_screen.h

@@ -0,0 +1,11 @@
+/*
+ * Copyright (c) 2020 Peter Johanson <peter@peterjohanson.com>
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+#include <lvgl.h>
+
+lv_obj_t *zmk_display_status_screen();