Add actions to increase/decrease slideshow delay

[nahoj]

Hi! I've been using feh for a while and I find it very handy.

I was wondering if it would be possible to add a possibility to change the slideshow delay after it's started, which is something that I often want to do. I'm thinking 2 bindable actions such as slideshow_delay_plus and slideshow_delay_minus, similar to reload_plus and reload_minus.

I appreciate your work on this app, thank you.