PresentationWidget: Fix screen switching at runtime

Merged David Hurka requested to merge davidhurka/okular:fix-screen-switching into release/20.08

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 (merged) 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.

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