Also allow selecting just a single connection.

This avoids issues with initialisation order.

We no longer send FIRs.

This is simpler and gets rid of ErrKeyframeNeeded.

This avoids a race condition since the delete messages are sent
asynchronously.

If simulcast is set to auto, disable simulcast when there are just
two users in the group.

This avoids code duplication in the various addUpMedia functions,
and makes it possible to replace a stream without reopening the
stream's source.  This will be required in order to change the
simulcast envelope.

Now that down streams have labels, we need to distinguish on the
stream direction.

Delete the stop video button, it is now redundant.  Panic remains.

Don't maintain loss, which is too error-prone.  Instead, maintain
expected and received packet counts, and compute loss from that.