tube-dbus: check if the tube has been requested to determine if we need to offer it
tube-dbus: check if the tube has been requested to determine if we need to offer it

If we rejoin a muc containing old D-Bus tubes we previously created they
now have the "Local Pending" state as expected instead of "Not Offered".
This fix b.fd.o #23678 as such tubes are now listed in
Tubes.ListTubes().

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