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/bindings/behaviors/zmk,behavior-keymap.yaml

@@ -1,8 +0,0 @@
-# Copyright (c) 2020, Pete Johanson
-# SPDX-License-Identifier: MIT
-
-description: Keymap Behavior
-
-compatible: "zmk,behavior-keymap"
-
-include: zero_param.yaml