PresentationWidget: Fix screen switching at runtime

Screen switching from inside presentation mode currently does not work correctly.
The reason is that the widget geometry() shall be moved
*while* the widget is configured as fullscreen.
This patch temporarily disables fullscreen, so the screen can be switched.

You can check it with release 20.04:
Screen switching from within presentation mode will not work.
Set the widget to windowed (e. g. Alt+F3 or right-click in task manager),
screen switching will work now.

!233 shall port PresentationWidget to QScreen in release 20.12,
so this is just a compact workaround without much intelligence.
The drawback is that custom window rules might get confused when the
screen geometry changes for some reasons.