Get rid of "[audio|video][On|Off]" datachannel events

[fancycode]

As just discussed offline with @Ivansss : currently the UI sends videoOn/videoOff and audioOn/audioOff events through the datachannels to all connected clients to notify about what is available and what is currently sending.

This is not necessary as you can get the information on media types from the offer/answer, and get the state (paused/running) from the streams. In order to remove duplicate functionality and simplify the code, these events should be removed.

[fancycode]

Also this would mean that any other client (mobile apps, SIP bridge, etc) would have to implement these events to make the (web) UI work correctly.

[nickvergessen]

I hope this does not interfere with #282 ?

[fancycode]

Not from what I understand. This issue is about detecting if another peer is sending audio / video, #282 is about disabling audio / video of other peers.

[fancycode]

What is the reason for the "signaling" label? There are no changes required for the signaling, all information should already be available from the peerconnections / streams.

[nickvergessen]

Increased priority as data channels are not supported in Edge 😢

[fancycode]

We could use the inCall flags to signal this information. They already contain bits for audio and video.