Add options to disable some/all buttons when hovering over thumbnails

Summary:
Adds three options:

  1) Show all (previous behaviour, now default)
  2) Show only selection buttons (hide fullscreen and rotate buttons in Browse View)
  3) Hide all buttons

This option affects Browse View, Image View Thumbnail Bar, and the Importer tool `gwenview_importer`.

FEATURE: 164847

Reviewers: rkflx, ngraham

Reviewed By: rkflx, ngraham

Subscribers: ngraham

Differential Revision: https://phabricator.kde.org/D10421