Throw an exception if assistant's performed sequences start incorrectly.

The first assistant's performed sequence must be a chordSequence
(not a restSequence) that is at the startMarker's msPosition.
Reorganized volume and aftertouch/channel pressure

The Assistant performer no longer looks for volume
messages in SVG-MIDI files.
(None of the existing scores contains volume control
info, and I am about to remove them from the
SVG-MIDI specification (and Moritz' score writing
software.)
In performances without a live performer: the AP sets
each track's volume to 125 before the performance
begins. No further volume messages are sent during
the performance.
In performances _with_ a live performer:
There is a new option "po.pressure.minimum=" that
can be set with the other performance options in
assistantPerformer.html. If pressure is mapped to
volume, this is the volume value to which all tracks
are initialised before the performance begins. If the
option is not set the volumes are initialised to 127.
If the option is mapped to some other control,
those controls are initialized to that value, and
volume is initialised to 127.
During the performance, pressure values (aftertouch
or channel pressure) are sent to the mapped
controller on all performing tracks, whereby the
pressure values (range 0..127) are mapped to the
range po.pressure.minimum..127.
Control substitution selectors are now visible/hidden

The selectors (in the performers options on the front
page) are now visible/hidden. They were previously
enabled/disabled.
Minimum pressure setting in main options panel

The minimum pressure setting, set by the value in the
score's id string in assistantPerformer.html, is now
available in the main options when either of the
pressure checkboxes is checked. The performer can
thus override the default value in the html file.
An alert is displayed if the setting is illegal when the
start button is pressed. (The same is now true for the
speed factor.)
Live performances are no longer set 'playing' when
the start button is pressed.