Detecting environment-facing (i.e. backside) cam

[AVGP]

This addresses the problem explained in #1

What it currently does is:

• it goes through the list of all sources for GetUserMedia
• it checks if a source is a camera and faces the environment (i.e. a backside camera)
• if it is, it starts capturing from this source.

I will amend it a bit, so that it is more defensive in terms of the availability of the MediaStreamTracks API and an environment-facing camera.

[AVGP]

Now it falls back to the getUserMedia call without a stream ID if

• no backfacing cam was found
• MediaStreamTracks or MediaStreamTracks.getSources isn't supported