Contact popovers

Enhances the contact popover to display an avatar, contact name and source email address, adds support for starring contacts, support for saving and opening contacts in the Contacts app (once gnome-contacts!66 (closed) lands), and allows updating the load-remote-images pref for senders not in the desktop address book.

This integrates further with Folks for displaying desktop address book information: Uses display name from desktop contacts when displaying email addresses in the conversation viewer and automatically load remote images for senders in the desktop addresss book.

Ticks some more items off of #133 (closed) and resolves #135 (closed)