background: add support for gif wallpapers

[Gitkubikon]

No description provided.

[PixelKhaos]

I did something similar, but scrapped it as it got messy since I didn't start with gif support, but rather video with mpvpaper. Was planning to give it another spin, but essentially mpvpaper just by default works perfect to run on top of the static wallpaper, and under other components in the correct z-index.

so what I did was just modify the picker to distinguish animated wallpapers from static with an icon, and when setting an animated one, call mpvpaper with with path and loop, as well as store it so that after restarts etc it would run mpvpaper again.

Taking a look at it, but I think there's some things you missed, I'll PR yer branch if I find anything :)