Add support for fetching group changes on the fly
So in case a user was not part of a group and it becomes (using Skype
UI), then BitlBee is now aware of this.
NOTE: current it seems the Skype part of this is broken. Use case: adding a
user to group #16 which already has a user.
<< GROUP 16 NROFUSERS 2
GET GROUP 16 NROFUSERS
<< GROUP 16 NROFUSERS 0
So it notifies about the number of users changed but I can't query the user
list as the number of users is then 0 - till Skype is not restarted.
