Desktop contacts integration

Geary should load desktop contacts via libfolks, so that contacts stored in EDS are available by default.

There's a a few things to do for this:

• Establish a folks session on startup
• Search folks for matching names and email addresses for autocomplete in the composer
• Lookup avatars in folks for conversations
• Lookup avatars in folks for notifications
• Lookup and show avatars in account settings
• Show images by default from senders found in folks
• Provide a means of adding/editing/removing contacts (via launching an app like Contacts?)
• Remove Gravatar support
• Show "initials tiles" instead of the stock avatar photo