UI: Restore frontend API events being dispatched during initialization

The frontend API events "OBS_FRONTEND_EVENT_SCENE_CHANGED" and
"OBS_FRONTEND_EVENT_PREVIEW_SCENE_CHANGED" are not dispatched if
disableSaving has a value > 0. Prior code decremented disableSaving to
0 globally (regardless of whether saving was necessary, which is only
the case if a new collection is created as a fallback for a missing
scene collection).

Thus the updated code exhibits different event dispatch behavior then
old code which some plugins might have relied on. To avoid introducing
new behavior, explicitly create the environment to allow the dispatch
to take place and remove/refactor the events in a later version.

UI/window-basic-main.cpp

@@ -2192,6 +2192,15 @@ void OBSBasic::OBSInit()
 			SetupNewSceneCollection(sceneCollectionName);
 			disableSaving++;
 		}
+
+		disableSaving--;
+		if (foundCollection || configuredCollection) {
+			OnEvent(OBS_FRONTEND_EVENT_SCENE_COLLECTION_LIST_CHANGED);
+			OnEvent(OBS_FRONTEND_EVENT_SCENE_COLLECTION_CHANGED);
+		}
+		OnEvent(OBS_FRONTEND_EVENT_SCENE_CHANGED);
+		OnEvent(OBS_FRONTEND_EVENT_PREVIEW_SCENE_CHANGED);
+		disableSaving++;
 	}
 
 	loaded = true;