Revise MIDI clock input synchronization

[izzyreal]

vmpc-juce currently does not forward MIDI clock input messages to mpc, see https://github.com/izzyreal/vmpc-juce/issues/87.

Apart from that, there is only a partial implementation in mpc's MpcMidiInput:

    if (syncScreen->in == index && syncScreen->getModeIn() != 0)
    {
        switch (mce->getStatus())
        {
        case ShortMessage::START:
            sequencer->playFromStart();
            break;
        case ShortMessage::CONTINUE:
            sequencer->play();
            break;
        case ShortMessage::STOP:
            sequencer->stop();
            break;
        case ShortMessage::TIMING_CLOCK:
            break;
        }
    }

So let's implement the TIMING_CLOCK case. One way to derive BPM from the current timing can be found here: https://forum.arduino.cc/t/read-midi-clock/1022060/14 But its correctness still needs to be verified.

Apart from that, we should revise behaviour of START, as per http://midi.teragonaudio.com/tech/midispec/seq.htm. In particular: "In practice, most masters allow a 1 millisecond interval inbetween the MIDI Start and subsequent MIDI Clock messages in order to give the slave an opportunity to prepare itself for playback."

[izzyreal]

WIP https://github.com/izzyreal/mpc/tree/midiclock-input2