UI: Use WaitConnection() when adding scenes

Instead of queuing AddScene to add the new scene to the list box,
ensures that the UI has added the new scene to the list box when the new
scene is added.  This ensures synchronization between the calling thread
and the UI thread in this particular case.

(Jim: Added detailed commit message description)

Closes obsproject/obs-studio#1226

UI/window-basic-main.cpp

@@ -2880,7 +2880,7 @@ void OBSBasic::SourceCreated(void *data, calldata_t *params)
 
 	if (obs_scene_from_source(source) != NULL)
 		QMetaObject::invokeMethod(static_cast<OBSBasic*>(data),
-				"AddScene",
+				"AddScene", WaitConnection(),
 				Q_ARG(OBSSource, OBSSource(source)));
 }