TelepathyClient: show notifications for presence changes

TelepathyClient: show notifications for presence changes

Fetch the names of the user's "subscribed" contacts, and use the
SimplePresence interface to watch for available/away/busy/etc messages
and create notifications for them.

Currently we display notifications when switching between "available"
and "offline"/"extended away", but when switching between "available"
and "away"/"busy" we just add the information to the chat window
without popping up a notification, to avoid spamming the user with
"Bob's screensaver activated" messages.

https://bugzilla.gnome.org/show_bug.cgi?id=611613