[MainWindow] Navigate through mouse forward/back buttons

Test Plan:

1. Open a folder in Gwenview
2. Click on image in
3. Navigate over images through forward/back mouse buttons (they are buttons on left side of the mouse)

Reviewers: ngraham, Gwenview

Reviewed By: ngraham, Gwenview

Subscribers: muhlenpfordt

Tags: Gwenview

Differential Revision: https://phabricator.kde.org/D14583