chatroom-manager: save file when only the 'favorite' prop is changed
chatroom-manager: save file when only the 'favorite' prop is changed

Needed to save favorite changes from empathy-chat as it only changes this
property.

https://bugzilla.gnome.org/show_bug.cgi?id=678983

GNOME/telepathy-account-widgets,GNOME/telepathy-account-widgets,Distrotech/telepathy-account-widgets,Distrotech/telepathy-account-widgets,GNOME/telepathy-account-widgets