Contact contextual popovers

The GTK widget conversion branch (Bug 765516) introduces contextual popovers for contacts, revealed by clicking on their email addresses. These are quite minimal at the moment however, and could be greatly expanded.

Some excellent, related design work has already been done for Polari: https://wiki.gnome.org/Design/Apps/Potential/Polari/ContextualPopup, and we would be looking to redesign and expand the existing popovers in a similar way for Geary.

Some requirements:

• Display contact name, avatar, important star, email address(es), remote image load permission
• Allow users to edit these details
• Allow users to launch contact-related actions (send email, search for messages from, allow/disallow remote image loading)

Initially, editing any of these details would simply update the user's contact entry in the Geary database.

For contacts accessed via Folks/EDS (#133 (closed)) we would also want to allow people to open the contact in Contacts, or add them to Contacts if not already present.