UI: Implement per-scene transition overriding

Allows the ability to override what specific transition a scene may use
when transitioning to it.

(Original proposal by cg2121, reworked by Jim)

Closes jp9000/obs-studio#1052

UI/window-basic-main.cpp

@@ -3470,6 +3470,11 @@ void OBSBasic::on_scenes_customContextMenuRequested(const QPoint &pos)
 		popup.addSeparator();
 		popup.addAction(QTStr("Filters"), this,
 				SLOT(OpenSceneFilters()));
+
+		popup.addSeparator();
+
+		QMenu *transitionMenu = CreatePerSceneTransitionMenu();
+		popup.addMenu(transitionMenu);
 	}
 
 	popup.exec(QCursor::pos());