Fix AvatarCache to be async and xplat

This removes the System.Drawing dependency.

(Also, disable XML docs, not needed and causes warnings).
Add Nickname/Avatar settings
Add Gravatar support to AvatarCache
Add AsyncValue for async value conversions
Add AvatarConverter
Add persona setup window
Launch persona setup if not previously setup
[Tests] Use mock context in AudioEngineTests
Nuke GTK client
Nuke CLI client