Reworked MIDI event handling in InstrumentTrack and renamed MIDI classes

The MIDI event handling in InstrumentTrack was complex and buggy. It has
been simplified now such that processInEvent() tries to handle note on,
note off and key pressure events. The actions taken should result in
equivalent calls to processOutEvent() by NotePlayHandle instances. The
processOutEvent() function sends according MIDI events to the attached
instruments. All unhandled MIDI events are directly forwarded to the
instrument in processInEvent().

It's possible that some corner-cases are not handled yet with the new code
and we have regressions now.

Furthermore renamed midiTime/midiEvent to MidiTime/MidiEvent to match
coding style.

Closes #72.
InstrumentTrack: removed option for disabling PianoView visual feedback

Probably never used by anyone and just makes code unneccessarily complex.
InstrumentTrack: removed option for disabling activity indicators

Probably never used by anyone and just makes code unneccessarily complex.
InstrumentTrack: do not set state of piano in processInEvent()

As processInEvent() is responsible for starting/stopping notes, we do
not have to set the state of the piano. This is done in processOutEvents()
already.
InstrumentTrack: work with transposed MIDI event and key in processOutEvent()

In processOutEvent() initialize and use a MidiEvent object with transposed
key as well as the key variable.