Adjusted the number of tracks for Song Six
Moved score constants into ap/ScoreConstants.js

Created the file ap/ScoreConstants.js containing
constants for each of the displayable scores.
This make it easier to change these constants as
necessary (especially the number of channels,
when this alone is changing), since they are no
longer embedded deep in the Controls.js code.
Score Infos are now all set in assistantPerformer.html

Deleted ScoreConstants.js.
Controls.js.setScore() now analyses the id of the option selected
in the scoreSelectionControl -- which is defined in
assistantPerformer.html.
This is a much better solution, since each selectable score's
name, nPages and nTracks is now all defined in the same place.
There is no longer any need to change Controls.js when a
score is added or changed in assistantPerformer.html.
Also: selecting "choose a score" in the score selector now
removes the displayed score.
Minor bug fixes in assistantPerformerStyleSheet.cs

Added pages 4-7 of Song Six.
The startRuntimeButton's text now changes.

To complement the disabled status of the button, its
text now changes to one of the values 'not ready',
'Start' or 'Running'. 'Start' is displayed whenever
the button is enabled..
I tried very hard to display a 'wait' cursor, change
the button's text to 'processing...', or display an
image somewhere during the button's onclick() function (i.e. while the MIDI sequence is being
constructed), but without success.