The functionality of Contacts should be a core part of Connection.
Proposed migration path:
* add GetContactAttributes and ContactAttributeInterfaces to the core Connection interface, with the same signature and stuff
* flatten TpContactsMixin into TpBaseConnection
* make similar changes in telepathy-python
* when those changes are old enough to rely on, change TpContact to use the core Connection interface instead of Contacts
* deprecate the Contacts interface
* in Telepathy 1.0 (Bug #23148), delete the Contacts interface
Telepathy 1.0 is relatively imminent and won't be backwards-compatible anyway, and porting will be straightforward, so let's just merge these interfaces at that point.