session: add way to get session id for specific conversation
    
    Right now, the only way to find out the session id of an opened
    (but not started) session is a parameter in the session-opened
    handler.
    
    This commit adds a new function
    gdm_session_get_conversation_session_id that returns the session-id
    for a specified opened conversation.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=744764