This to prevent compilation issues between runs with and without precompiled
headers. Also remove the headers from the rest of the code base as they are
not needed there anymore, although they do relatively little harm.
MCI MIDI is poorly supported on newer versions of Windows and can cause large
delays at the beginning of tracks.
The new driver is based on a from-scratch reader for Standard MIDI Files. This
should be re-usable in other music drivers too, and can allow for finer control
of playback in general.
It also provides a better framework for reading MIDI data from other formats
than just SMF.