Add photo hash.

The photo hash is hold by QyncBuddy as a QByteArray. Hope it is not a
problem.
Data structure refactor again.

1. It was so stupid to have a QyncCommonBuddy. getGroup is a necessary
method for the backend. And its ownership was not clear before, backend
always met a deleted pointer.
2. Implement the sipe_backend_buddy_find_all.
3. Still, it would be a good idea to buffer all the buddies and never
release them. Maybe do this in the future.