Add a "Mirror mode" to the speaker view

[panglesd]

The speaker view tries to maintain a synchronized presentation: the step number, but also a few other things like the drawings.

However, not only it is easy to add state to the presentation that would not be synchronized (eg an x-ocaml block), but sometimes in a presentation one shares other things than the presentation (eg, makes a demo of another tool).

This PR adds a "mirror" mode for the speaker view, which simply mirrors the other screen you share with your audience in your own screen, where you have the notes.

cc @jonludlam