group_namelistchange(): for gui reasons, divulge peernumber and change type

tox.*, Messenger.*, group_chats.*:
- expand interface of callback by peernumber/change

groupchats.c:
- call callback in addpeer/delpeer/setnick with peernumber and flag

nTox.c:
- print_groupchatpeers(): print as many names on a line as possible, not one peer per line
- print_groupnamelistchange(): only print the change, if possible (i.e. "new peer", "peer's new name")
- added command "/p" to print the list of peers