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/src/event_manager.c

@@ -0,0 +1,32 @@
+
+#include <zephyr.h>
+#include <logging/log.h>
+
+LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
+
+#include <zmk/event-manager.h>
+
+extern struct zmk_event_type* __event_type_start[];
+extern struct zmk_event_type* __event_type_end[];
+
+extern struct zmk_event_subscription __event_subscriptions_start[];
+extern struct zmk_event_subscription __event_subscriptions_end[];
+
+int zmk_event_manager_raise(struct zmk_event_header *event)
+{
+    int ret;
+    struct zmk_event_subscription *ev_sub;
+    for (ev_sub = __event_subscriptions_start; ev_sub != __event_subscriptions_end; ev_sub++) {
+        if (ev_sub->event_type == event->event) {
+            ret = ev_sub->listener->callback(event);
+            if (ret) {
+                LOG_DBG("Listener returned an error: %d", ret);
+                goto release;
+            }
+        }
+    }
+
+release:
+    k_free(event);
+    return ret;
+}