Changed the title of "Song 6 Sketch" to "Song Six"
Realised silent live performer option

Added a new option check box to the live performer's
options. The performed track can now be silent in live
performances.
In Song Six, I envisage having default sounds in the
soloist's track - which will play back in the default
performance - but that in live performances, the solo
part will be performed live.
Changed how to toggle the sounding/silent soloist.

Removed the "silent" checkbox from the upper options panel. The soloist in an assisted
performance is now switched between sounding
and silent (in the same way as for all other tracks)
by toggling its track control.
Bug Fix: The input device's MIDI input event handler
is now added and removed when setting the
performance state (playing, paused, stopped).
Previously, the event handler was added whenever
an input device was selected in one of the device
selectors.