Set transient parent for system tray menus

Summary: BUG: 381174

Test Plan:
Had debug in QtWayland
Clicked on systray
Had parent, menu was in the right position

Reviewers: #plasma, mart, graesslin

Reviewed By: #plasma, mart, graesslin

Subscribers: apol, graesslin, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D8338

applets/systemtray/systemtray.cpp

@@ -221,6 +221,8 @@ void SystemTray::showPlasmoidMenu(QQuickItem *appletInterface, int x, int y)
     }
 
     KAcceleratorManager::manage(desktopMenu);
+    desktopMenu->winId();
+    desktopMenu->windowHandle()->setTransientParent(appletInterface->window());
     desktopMenu->popup(pos.toPoint());
 }