addTrackButton (track) {
    const
      t = this,
      label = track.label || i18n.t(mejs.language.codes[track.srclang]) || track.srclang;

    // trackId is used in the value, too, because the "none"
    // caption option doesn't have a trackId but we need to be able
    // to set it, too
    t.captionsButton.querySelector('ul').innerHTML += `<li class="${t.options.classPrefix}captions-selector-list-item">` +
      `<input type="radio" class="${t.options.classPrefix}captions-selector-input" ` +
      `name="${t.id}_captions" id="${track.trackId}-btn" value="${track.trackId}" disabled>` +
      `<label class="${t.options.classPrefix}captions-selector-label"` +
      `for="${track.trackId}">${label} (loading)</label>` +
      `</li>`;
  }
Adds a new track button.
@param {object} track