Remember fullscreen mode and restore on next start

Summary:
Currently Gwenview always starts in normal (non-fullscreen) mode
independent of its state on previous quit.
This patch saves the fullscreen mode and restores it on next start.

BUG: 383093

Test Plan:
Quit Gwenview in normal/fullscreen mode and check if it restarts in
previous mode.
Commandline option --fullscreen overrides the saved state.

Reviewers: #gwenview, rkflx

Reviewed By: #gwenview, rkflx

Subscribers: rkflx, ngraham

Differential Revision: https://phabricator.kde.org/D10853