Fix/redesign PS and sugar.presence's tracking of buddy handles

[morgs]

Activities have a get_buddy function which calls Telepathy directly to resolve a buddy handle.

My attempt to provide a buddy-handle cache in sugar.presence didn't work well in that once a buddy left, we remove the mapping, although we depend on resolving the mapping after the fact for UI purposes ("morgs left the chat"...)

This needs a rethink so that we can replace the activity use of TP directly for this, and yet provide what the activity needs.

[morgs]

Patch to add buddy-handle resolution in subclass of TubeConnection

[erikos]

Let's see how this has been changed with the reorganization of the code.