Transport icons

Use parent gateway component for avatar transport icons.

Disco cat=gateway, type=XXX means "I am a component that allows talking to the XXX service", but contacts themselves have no standard way of saying "I use XXX". There were some talks in the XSF room about that, and if "bridged contacts" start advertising "disco cat=gateway" we probably will break clients, including cheogram.

This MR has a caching/display update issue: for all avatars to appear in the chat list rows and roster rows, gajim needs to be exited/relaunched. Some pointers on how to fix that would be great! Maybe we need to store both avatar SHA and the gateway type to make sure cached avatars are valid.