Added the processing of MIDIEvents in the MIDI drivers

The temporary debug methods MIDIDriver::sendNoteOn and
MIDIDriver::sendNoteOff have been replaced by MIDIDriver::processEvent.

The new method is implemented in both available MIDI drivers.
MIDIDriverALSA already maps the MIDIEvent to the corresponding ALSA API
methods.

MIDIDriver::processEvent is called in
JoystickToMIDIMapper::handleJoystickEvent which now sends different
notes for different buttons.