qt: avoid using (transient) parentless `QMenu`

Qt does not allow mixing Qt Quick and Qt Widgets (QWidget does not accept raw QWindow as its parent). We should set the transient parent manually to the main (quick) window to hint the window manager that the menus are connected to the application (main window).

In most platforms, menus are a window. However, I still check in newly introduced VLCMenu that the menu is indeed a window before setting the transient parent.