New functions for (currently unused) mouse side buttons

[Dreamer]

In View mode and fullscreen the mouse side buttons have no function. I have few suggestions:

- Add automatically the other function than mouse wheel (prev./next file / zoom)

- Custom option for mouse side buttons (prev./next file / zoom / fit image to window / 100% zoom)

I think the option to swap the buttons might be needed too, because the position of those buttons is not always the same.

Fully customizable mouse shortcuts for all modes would be the best, of course.