[v3] spacing inside of maligngroup is incorrect

The contents of <maligngroup> elements don't get the correct spacing. This is due to the fact that the MmlMaligngroup class is a subclass of AbstractMmlNode rather than AbstractMmlLayoutNode. An example is

<math>
  <mtable>
    <mtr>
      <mtd>
        <maligngroup>
          <mi>x</mi>
          <mo>+</mo>
          <mn>1</mn>
        </maligngroup>
      </mtd>
    </mtr>
  </mtable>
</math>

where the spacing around the plus sign is lost.