* Adds IT MIDI macro configuration handling to the IT loader.
* Adds a player parser for IT MIDI macros and support for several
MIDI macro variables supported by Impulse Tracker.
* Adds support for the IT set parameterized macro, MIDI macro,
and smooth MIDI macro (OpenMPT extension) effects.
* Adds six missing OpenMPT IT MIDI macro tests and updates the
OpenMPT IT readme. Also adds a new test for smooth MIDI macros.