Summary:
When switching to fullscreen view mode the statusbar is hidden but the
`KToggleAction` is not updated. This leads to unhiding the statusbar
after applying config dialog and forcing the user to press {key F3}
twice to show the statusbar.
This patch keeps visibility and action state in sync by using persistent
`GwenviewConfig` values for browse mode and normal/fullscreen view
modes like sidebar.

Test Plan:
Switch browse/view, normal/fullscreen modes.
The statusbar state should always restore to previous setting.
Three different states are (re)stored:
* Browse mode (always equal in normal/fullscreen mode)
* View mode normal (windowed) mode
* View mode fullscreen mode

Reviewers: #gwenview, rkflx

Reviewed By: #gwenview, rkflx

Subscribers: rkflx

Differential Revision: https://phabricator.kde.org/D10577