Enable back mouse button to go back in settings (like in cosmic-files)

[Tichy]

Knowning the behaviour of most applications, we are used to go to the previous screen of an application by using the back button of the mouse (when present). This works, for example, also in cosmic-files.

How to reproduce:

1. Open cosmic-settings
2. Click on Desktop
3. Click on Wallpaper
4. Press the back button on the mouse
5. You'll expect to be brought back to the Desktop screen, but you are still on Wallpaper