showOverlay() {
    const overlay = m('div', null, {
      className: 'form-builder-overlay',
    })
    document.body.appendChild(overlay)
    overlay.classList.add('visible')

    overlay.addEventListener('click', ({ target }) => this.closeConfirm(target), false)
    document.addEventListener('keydown', this.handleKeyDown, false)

    return overlay
  }
Adds overlay to the page. Used for modals.
@return {HTMLElement} DOM Object