constants.py: update from telepathy-gabble, mostly
constants.py: update from telepathy-gabble, mostly

However, revert the change to the final ContactCapabilties namespace:
Salut only implements draft 1.

freedesktop-unofficial-mirror/telepathy__telepathy-salut,freedesktop-unofficial-mirror/telepathy__telepathy-salut,freedesktop-unofficial-mirror/telepathy__telepathy-salut,freedesktop-unofficial-mirror/telepathy__telepathy-salut