Disable auto saving window size in fullscreen mode

Summary:
If gwenview quits while in fullscreen mode the window size is
automatically saved and gwenview restarts in maximized state.
This patch disables saving window state before switching to
fullscreen mode.

Test Plan:

1. Open gwenview in windowed, not maximized mode
2. Switch to fullscreen mode (e.g. F11)
3. Quit gwenview by Ctrl+Q
4. Restart gwenview

-> Window size should be equal to step 1 and not maximized

Reviewers: rkflx

Reviewed By: rkflx

Differential Revision: https://phabricator.kde.org/D10207