Roster: Use icons from IconTheme for Roster

With this Gajim loses the ability to draw animated GIFs in the Roster

GIFs have numerous drawbacks:

- We cant add them to the IconTheme, which means we need a different interface to load them
- The IconTheme scales all icons we load for the current scale (HiDPI)
- The animation causes many updates to the Roster, and causes high cpu usage

Fixes #8814, #8655