Using Scheme to display the list of all supported midi instruments

Supported MIDI instruments are defined in the file scm/midi.scm. Using scheme, one can easily create a lilypond file displaying all supported midi names automatically.
The way to do this is to load the midi.scm file and then creating a \markup for each midi instrument definition. This is done by a markup-list function written in Scheme. The resulting list of markups can then be displayed using \markuplist.