[Notifications] Set transient parent for file menu

Required on Wayland so it can be properly positioned.

BUG: 387597
FIXED-IN: 5.18.0

Differential Revision: https://phabricator.kde.org/D26943

applets/notifications/filemenu.cpp

@@ -178,6 +178,8 @@ void FileMenu::open(int x, int y)
         pos = m_visualParent->mapToGlobal(QPointF(x, y)).toPoint();
     }
 
+    menu->winId();
+    menu->windowHandle()->setTransientParent(m_visualParent->window());
     menu->popup(pos);
 
     m_visible = true;