AUDIO: Change several fake MidiDrivers to MidiDriver_BASE subclasses

Many engines follow the advice in audio/midiparser.h and create a
"pseudo-MidiDriver" subclass. But MidiParser really only needs a tiny
subset of the MidiDriver capabilities, namely those found in
MidiDriver_BASE. So we try to subclass from that whenever possible; this
allows us to remove many stub methods, and enables further future
simplifications.

engines/agos/midi.cpp

@@ -84,10 +84,6 @@ int MidiPlayer::open() {
 	return 0;
 }
 
-bool MidiPlayer::isOpen() const {
-	return _driver && _driver->isOpen();
-}
-
 void MidiPlayer::close() {
 	stop();
 //	_system->lockMutex(_mutex);