Remember fullscreen mode and restore on next start
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.
Quit Gwenview in normal/fullscreen mode and check if it restarts in
Commandline option --fullscreen overrides the saved state.
Reviewers: Gwenview, rkflx
Reviewed By: Gwenview, rkflx
Subscribers: rkflx, ngraham
Differential Revision: https://phabricator.kde.org/D10853