YtsgRoster: added convenience service-added and service-removed signals
YtsgRoster: added convenience service-added and service-removed signals

These convenience proxies of YtsgContact::service-added/removed signals (so
that applications need not to worry about the contact signals, that are not
really that interesting in most cases).

freedesktop-unofficial-mirror/ytstenut__ytstenut-glib,freedesktop-unofficial-mirror/ytstenut__ytstenut-glib