Don't show the misleading "user cancelled" error, there are corner cases
in which a different error can be manifested.

When addCancellableListener was introduced we missed to stub it here.

When accepting a new offer.

* Remove support for legacy endpoint based signaling and make source-name signaling as the only signaling mode. The legacy screensharing mode will no longer be supported. The client will still be able to process remote presence sent in the old format to support interop with very old mobile clients and jigasi that do not support source-name signaling.
* remove code related to presenter mode.

It does the minimum necessary to function without the complexity of the
main one.

These have becomee necessary with the introduction of multi-stream.

Regression introduced in
5c9b905111

It makes more sense to call it pcConfig since it is the RTCConfiguration object passed to the WebRTC peerconnection.

Fixes Spot not being able to use the wireless screen sharing feature.

* feat: Set degradation preference on video senders
Use 'maintain-framerate' for camera tracks and 'maintain-resolution' for desktop tracks

* ref: apply cached video contraints and degradation preference on unmute
Since the sender encodings are available only after the renegotiation is complete (on chrome/safari), constraints/preferences have to be applied after renegotiation is complete.

...to prevent a crash. The functionality that depends on it
is not required by ProxyConnectionPC.

* feat(screenshare): support remote wireless screenshare

- ProxyConnectionService is exposed and meant to be the
  facade for creating and updating a peer connection with
  another peer, outside of the muc.
- ProxyConnectionPC wraps JingleSessionPC so the peer
  connection handling can be reused.

* attempt to make more configurable