This also should fix some regressions from the previous
commits, related to MidiParser's either being leaked,
or being deleted and then used again (i.e., crashing).
I tested as many games as I had available, but further
testing of all affected engines is called for anyway.
This code is currently not complete, but contains enough code to
allow several engines to switch their pseudo MidiDrivers to be
based on this class, greatly reducing code duplication.
