jingle-session.c: refactor content creation code
jingle-session.c: refactor content creation code

Ziemin/telepathy-gabble,mlundblad/telepathy-gabble,jku/telepathy-gabble,community-ssu/telepathy-gabble,mlundblad/telepathy-gabble,community-ssu/telepathy-gabble,Ziemin/telepathy-gabble,Ziemin/telepathy-gabble,jku/telepathy-gabble,jku/telepathy-gabble,community-ssu/telepathy-gabble,mlundblad/telepathy-gabble,Ziemin/telepathy-gabble,community-ssu/telepathy-gabble