qt: fix window destroyed before the vout window in Wayland compositor

fix: #28760 (closed)

note that !5939 (merged) is still needed for it to work properly

this is marked as Draft as I'm not sure the solution is quite right, I can't warranty that the QML view content as been effectively replaced nor that the QML resources are really released

m_guiUnloadingNeedContentClear test has been removed as calling setSource(QUrl()) a second time is basically a no-op