Major enhancement of performer's speed control

This enhancement is substantially complete and working,
but I am about to do some further tweaking...
This enhancement consists of:
assistantPerformer.html:
Deleted the Assistant's Duration options checkBoxes.
Added 1 a new performer's options selector, for selecting
the input control responsible for controlling speed, and
2 a corresponding text input determining the maximum
speed (re the score) that can be produced. Also
realised the corresponding "po." options in the Song Six
score selector option (in assistantPerformer.html).
Controls.js
Added the funtionality for the above new controls.
In particular, a new options.runtime.speed attribute.
Assistant.js
The speed factor is now calculated and sent to the
performing sequence whenever the performer sends
an appropriate control message. This happens
continuously, even while a sequence is playing.