Add new "piano roll" graphic

[lemiffe]

Consists of a set of 12 small lines across the horizontal area, increasing in height, which will represent the current note being played of one of the MIDI devices.

Map each note to the graphic using something similar to this in mutateGameState:

const keyColors = {
        'C': 0x9966FF,
        'D': 0xFF0000,
        'E': 0x00FF00,
        'F': 0x0000FF,
    };
    if (midiNotes.length) {
        state..item.. = keyColors[midiNotes[0].note.key] || 0x9966FF

But instead of changing the colour, we make it visible or not.