Create a PidginAvatar widget.

This does everything the existing code does, but trying to integrate right now
is kind of difficult. The plan is to use this in a new PidginInfoPane I have
started, but that change got very large so I just packed it into the end of
the existing info pane.

The only things that are not implement right now, are making menu items
insensitive and that's because we need to figure out a better want to handle
custom avatars for users.

Testing Done:
Ran locally.

Reviewed at https://reviews.imfreedom.org/r/528/