[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