core: process incoming changes for roaming contacts
If the user uses more than one client then updates to the buddy list
done in one client will be sent as NOTIFY/roaming-contacts+xml to the
others. The XML root is "contactDelta".
Added support for
- group renaming
- buddy alias update
- buddy added to group
- buddy removed from group
Not implemented yet
- new group
- group deleted
- buddy deleted
Introduced new backend API sipe_backend_buddy_group_rename() to support
renaming of groups. Supplied a functional implementation for purple and
dummy stubs for miranda & telepathy.