Implements a leaner avatar caching system. The avatar component
listens for uid filtering its id, which may be:
- conversation id
- account id
- contact uri

In response to the uid change, a the image source is updated with
a new image url invoking a fresh QQuickImageProvider query. With
this design, only the avatarregistry's uid mapping needs to be
updated when profiles are changed, and no longer should specific
avatar components receive manual source updates.

Gitlab: #466
Change-Id: Ie5313f5c187a0977ca51b890dd92187480a42537