[FEATURE] wallpaper

[theotheroracle]

i was requested to add wallpaper support, as in, ability to set a background for chats, i'll look into doing this :)

[Airyzz]

This is already somewhat supported by the custom theming system - we can set an image wallpaper for the app theme, however we cant specify a custom wallpaper on a per-chat basis

the theming system needs documentation, which I haven't got around to yet

[theotheroracle]

oh excellent