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/dts/bindings/zmk,keymap.yaml

@@ -7,6 +7,9 @@ child-binding:
   description: "A layer to be used in a keymap"
 
   properties:
+    label:
+      type: string
+      required: false
     bindings:
       type: phandle-array
       required: true