client: send out teardown signal before tearing down
client: send out teardown signal before tearing down

The advantage is that in the signal handler you get direct access to
information about what streams are about to get torn down (in the
GstRTSPClientState).

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=697686

thaytan/gst-rtsp-server,hsteinhaus/gst-rtsp-server,barisdemiray/gst-rtsp-server,hsteinhaus/gst-rtsp-server,Lachann/gst-rtsp-server,barisdemiray/gst-rtsp-server,thaytan/gst-rtsp-server,surround-io/gst-rtsp-server,surround-io/gst-rtsp-server,Lachann/gst-rtsp-server,Lachann/gst-rtsp-server,thaytan/gst-rtsp-server,surround-io/gst-rtsp-server,barisdemiray/gst-rtsp-server,hsteinhaus/gst-rtsp-server