Initial event manager work, and two first events.

* Add initial event manager implementation,
  roughly mimicking Nordic's API.
* Add `position_state_changed` and
  `keycode_state_changed` events.
* Hook up HID and keymap to new events
  instead of using behaviour global event
  crazy.

app/dts/behaviors/keymap.dtsi

@@ -1,9 +0,0 @@
-/ {
-	behaviors {
-		keymap_behavior: behavior_keymap {
-			compatible = "zmk,behavior-keymap", "zmk,behavior-global";
-			label = "KEYMAP";
-			#binding-cells = <0>;
-		};
-	};
-};