allow setting the jid used as initiator / responder

[fippo]

the some usecases (e.g. muc), the jid in the initiator field might have to be different.

[fippo]

we should also allow accessing that JID -- which is currently not possible

[fippo]

oh, turns out we currently set neither initiator nor responder.

[fippo]

that is somewhat tricky... we can set the local JID in the case of receiving a session-initiate but i am not sure what to do about an outgoing call. @legastero?