GabblePresenceCache: emit a signal on unsure -> sure transition
GabblePresenceCache: emit a signal on unsure -> sure transition

This also means we don't have to compare GTimeVals in order to see whether
we're sure yet.

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