Sort scenes alphabetically

Prior to implementing canvas suppor they were in the order they are in
the scenes dock.
After switching from obs_frontend_get_scene_names() to
obs_canvas_enum_scenes() this seems to not always be the case.

lib/utils/scene-selection.cpp

@@ -334,6 +334,7 @@ static QStringList getCanvasScenesList(obs_weak_canvas_t *weakCanvas)
 
 	QStringList list;
 	obs_canvas_enum_scenes(canvas, enumCanvasScenes, &list);
+	list.sort();
 	return list;
 }