user-accounts: Add methods to fetch avatars from online accounts

Along with a simple test utility that fetches my gravatar and the
local online accounts.

Based-heavily-on-a-patch-by: Bogdan Ciobanu <bgdn.ciobanu@gmail.com>
user-accounts: Add a new um-avatar-picker to replace um-photo-dialog

This is a new implementation using GtkStackSwitcher and composite
template widgets and friends that implements the new avatar picker
designs by Allan.

Based-heavily-on-a-patch-by: Bogdan Ciobanu <bgdn.ciobanu@gmail.com>
user-accounts: Replace um-photo-dialog with new um-avatar-picker