Make the de-facto "comment" and "contact" fields official.

This avoids flicker upon renegotiation.

Make it possible to attach filters dynamically.  Work around bugs
in Firefox.

If we send two offers back to back, the second answer will arive in
stable state, which will confuse us.  Instead, queue the second offer.

No user interface yet.

This avoids special-casing file streams.

Thanks to Jeroen van Veen for the debugging.

We used to call it on just the first track.  Also remove obsolete
remains of the onlabel callback.

Avoid formatting JSON multiple times.