webrtcbin connection-state always new because dtls-transport states don't change

Submitted by Jan Schmidt @thaytan

Link to original bug (#796896)

Description

GstWebRTCDTLSTransport doesn't ever change the "state" property from NEW, so the overall connection-state of webrtcbin never changes.

Part of the problem is that dtlssrtpbin doesn't expose enough API to report the state properly yet, but for now just switching to CONNECTED when on-key-set is fired.