Allow for adding/removing actions from menu

[markNZed]

I would like to customize the feh GUI menu to add options for next_img and prev_img With that feature a touchscreen could allow for a single tap to show the menu and then the user could tap again to exit or view next/prev_img It is already very close to replacing a digital picture frame. Thanks for all the work on feh!

I'm using Wayland so the system gesture of swiping from the top of the screen provides a way to exit the slideshow via the touichscreen, then the tap event can be used for next_img.

Maybe it would be easy to add double click as one of the button options ? Then a double click could be used tog exit the slideshow.