Add timestamps to position and behavior events (#147)

* Add timestamps to position events and behaviors.

- Take original event timestamps into consideration so nested tap-holds have proper timing.
- Add position and timestamp to keycode state changed event so the one-shot behavior can properly identify other keypresses and timings.
- Add timestamp to position events received from peripheral

* reduce number of arguments to behaviors

app/src/split/bluetooth/central.c

@@ -60,6 +60,7 @@ static u8_t split_central_notify_func(struct bt_conn *conn, struct bt_gatt_subsc
                 struct position_state_changed *pos_ev = new_position_state_changed();
                 pos_ev->position = position;
                 pos_ev->state = pressed;
+                pos_ev->timestamp = k_uptime_get();
 
                 LOG_DBG("Trigger key position state change for %d", position);
                 ZMK_EVENT_RAISE(pos_ev);