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/include/zmk/events/position-state-changed.h

@@ -0,0 +1,12 @@
+#pragma once
+
+#include <zephyr.h>
+#include <zmk/event-manager.h>
+
+struct position_state_changed {
+    struct zmk_event_header header;
+    u32_t position;
+    bool state;
+};
+
+ZMK_EVENT_DECLARE(position_state_changed);