Moved master volume control to performer options.

Confusingly, this used to be called maxVolume.
The TrackInit form now just contains real MIDI
settings. There is a comment on the Mono
Performer Options form about how the master
volume setting is going to work.
Controller values now saved explicitly.

In .mkss files, default values are now always
saved explicitly, never as empty string values.
The forms' behaviour has not changed --
default values are not displayed inside the
IntListControl, but are saved using a new
ValueAsString(int) function.
Handling of defaults changed in IntListControl

IntListControl boxes are now only empty if they
are all empty. This simplifies both the use of
dialogs and the way info is stored in .mkss files.