For one-on-one chats without a custom icon, the other user's avatar will
be used as the chat's icon.
A default chat icon was also added
(haiku/data/artwork/icons/WebsiteComments).
Add a program-wide avatar cache― whenever a contact or user's avatar has
changed, it gets cached in ~/config/settings/Caya/Cache/$account/People
(for contacts) or $account/Users (for everyone else).
ImageCache is now used, only for caching the kPersonIcon resource image―
it might be useful later for other commonly used images (status
indicators, etc).
Fixes#4
Jaidyn Ann
plfiorini