Fix thumbnail bar in View resetting to OFF when quitting in fullscreen

Summary:
Quitting Gwenview was saving the current state of the {nav Thumbnail Bar}, which is always
hidden in fullscreen, and therefore was always saving an OFF state to the config.
This patch saves the state of the {nav Thumbnail Bar} before going to fullscreen.

Test Plan:
Go to {nav View}, toggle {nav Thumbnail Bar} on, go to fullscreen, press {key Ctrl Q} to quit.
After launching Gwenview again, {nav View} {nav Thumbnail Bar} should be visible.

Reviewers: #gwenview, rkflx

Reviewed By: #gwenview, rkflx

Differential Revision: https://phabricator.kde.org/D10785