UI: Enable drag&drop reordering for scenes

The empty implementation for the QDataStream operators is necessary to fix
a Qt assertion: "QVariant::save: unable to save type
'std::vector<std::shared_ptr<OBSSignal> >' (type id: 1036)."

It doesn't seem like a full implementation of the stream operators would be
useful since the signal registration is very specific to the interaction
between the scenes and sources list, i.e. in case the scenes list ever accepts
scenes from sources (not 'obs_source's) other than itself (non InternalMove
drag&drop?) it would have to register those scenes with the libobs core which
should trigger the normal signal registration

obs/window-basic-main.cpp

@@ -133,6 +133,9 @@ OBSBasic::OBSBasic(QWidget *parent)
 	qRegisterMetaType<OBSSource>   ("OBSSource");
 	qRegisterMetaType<obs_hotkey_id>("obs_hotkey_id");
 
+	qRegisterMetaTypeStreamOperators<
+		std::vector<std::shared_ptr<OBSSignal>>>(
+				"std::vector<std::shared_ptr<OBSSignal>>");
 	qRegisterMetaTypeStreamOperators<OBSScene>("OBSScene");
 	qRegisterMetaTypeStreamOperators<OBSSceneItem>("OBSSceneItem");