UI: Add TBar controls to obs-frontend-api

Adds `obs_frontend_set_tbar_position` and `obs_frontend_release_tbar`,
which allow plugins and scripts to control the tbar in OBS.
This specific change is required for the `SetTBarPosition`
request to be added to obs-websocket.

UI/window-basic-main-transitions.cpp

@@ -915,6 +915,8 @@ void OBSBasic::TBarChanged(int value)
 	OBSSource transition = obs_get_output_source(0);
 	obs_source_release(transition);
 
+	tBar->setValue(value);
+
 	if (!tBarActive) {
 		OBSSource sceneSource = GetCurrentSceneSource();
 		OBSSource tBarTr = GetOverrideTransition(sceneSource);