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.