mavlink & system: add events interface

- sending protocol
- uorb event message & template methods for argument packing
- libevents submodule to send common events and handle json files
- cmake maintains a list of all (PX4) source files for the current build
  (PX4 modules + libs), which is used to extract event metadata and
  generate a json file

src/modules/mavlink/mavlink_receiver.cpp

@@ -288,6 +288,10 @@ MavlinkReceiver::handle_message(mavlink_message_t *msg)
 		handle_message_gimbal_device_information(msg);
 		break;
 
+	case MAVLINK_MSG_ID_REQUEST_EVENT:
+		handle_message_request_event(msg);
+		break;
+
 	default:
 		break;
 	}
@@ -2834,6 +2838,11 @@ void MavlinkReceiver::CheckHeartbeats(const hrt_abstime &t, bool force)
 	}
 }
 
+void MavlinkReceiver::handle_message_request_event(mavlink_message_t *msg)
+{
+	_mavlink->get_events_protocol().handle_request_event(*msg);
+}
+
 void
 MavlinkReceiver::handle_message_gimbal_manager_set_manual_control(mavlink_message_t *msg)
 {