Getter for SynthesiserVoice::currentPlayingMidiChannel

similar to the already existing getCurrentlyPlayingNote(),
a getter for the currentPlayingMidiChannel private member would be useful:

/** Returns the midi channel of the note that this voice is currently playing.
     Returns 0 if no note is playing.
*/
int getCurrentlyPlayingMidiChannel() const noexcept { return currentPlayingMidiChannel; }