Set a transient parent for SNI context menus

Summary:
Those had no transient parent set, so they got displayed somewhere, most of the
time on the wrong screen.

Test Plan:
Works fine for spotify and kteatime now, but certain applications
which trigger a LayoutChanged signal on the opened event have a Y offset.

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11584

applets/systemtray/systemtray.cpp

@@ -293,8 +293,9 @@ void SystemTray::showStatusNotifierContextMenu(KJob *job, QQuickItem *statusNoti
             }
         }
 
-
         KAcceleratorManager::manage(menu);
+        menu->winId();
+        menu->windowHandle()->setTransientParent(statusNotifierIcon->window());
         menu->popup(QPoint(x, y));
     }
 }