QLC+: Add null pixelKeys as empty heads

Extracted from #421, the following change will create every possible matrix position as QLC+ head (even "holes") which better visualizes non-rectangle matrices.

Before adding this to our QLC+ plugin, we should make a pull request to QLC+ that allows empty heads in the XML schema and, most important, hides empty heads from the 2D view.

  if (hasMatrixChannels) {
    for (const zLevel of mode.fixture.matrix.pixelKeyStructure) {
      for (const row of zLevel) {
        for (const pixelKey of row) {
          const channels = mode.channels.filter(channel => controlsPixelKey(channel, pixelKey));
          const xmlHead = xmlMode.element(`Head`);

          for (const ch of channels) {
            xmlHead.element({
              Channel: mode.getChannelIndex(ch.key)
            });
          }
        }
      }
    }
  }

Start by reviewing the QLC+ XML schema and its 2D view behavior, then compare them with the matrix export snippet in this issue. Done means QLC+ accepts empty Heads for matrix holes and hides those empty heads from the 2D view; the requested change is an upstream QLC+ pull request.