webrtc: add precise synchronization example

This example demonstrates a sender / receiver setup which ensures precise synchronization of multiple streams in a single session.

RFC 6051-style rapid synchronization of RTP streams is available as an option. See the Instantaneous RTP synchronization... blog post for details about this mode and an example based on RTSP instead of WebRTC.

Depends on:

• !1511 (merged)
• !1502 (merged)
• !1462 (merged)
• gstreamer!6119 (merged)
• gstreamer!6109 (merged)
• gstreamer!6110 (merged)
• gstreamer!6116 (merged)