Fix MUC Light affiliation change handling

[pwetrifork]

This pull requests fixes the following issues in MUC Light -xmppMUCLight:changedAffiliation:roomJID: callback:

• The callback was only invoked once per incoming affiliation change message even if the server reported changes for several occupants.
• The callback didn't include the JID of the occupant whose affiliation changed

The above fix is a prerequisite to introduce the last message correction handling module, one of the modules mentioned in the "Currently supported XMPP extensions" section in #993.

Technically, this pull request breaks XMPPMUCLightDelegate API. There are some good reasons to do so though:

• MUC Light may still be considered as having "semi-experimental" status
• The callback in its old form could misguide application developers leading to bugs where some affiliation changes go undetected