Add Qt6 CI support

Kai Uwe Broulik requested to merge work/kbroulik/qt6ci into master

Needed a minor fix in mountiso action:

In Qt6 QAction constructor takes a QObject* for its parent so it needs to be able to resolve that QWidget inherits from that.

@vkrause @felixernst

Edited by Kai Uwe Broulik

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