Remove commhistory-related API

With libcommhistory-declarative and recent refactoring there, it
no longer makes sense to base the telepathy handling API around
commhistory groups, nor to manage the group model as GroupManager
did.

All of those were removed in favor of ConversationChannel representing
exactly one telepathy channel directly, and ClientHandler turning into
a manager for channels.
Remove libcommhistory dependency
Update MessageContextProvider for property changes

Added the ObservedInbox property, and changed the API and value of
ObservedConversation to allow multiple groups to be observed
simultaniously.

Requires commhistory-daemon 0.4.7.