Fix determination of line-breaing parent element. #1548.

[dpvc]

Fix determination of line-breaking parent element. It was moving too high in some cases. E.g., in

  <mrow>
    <msqrt>
      ...
    </msqrt>
  </mrow>

the inferred mrow in the msqrt was stepping out to the msqrt (correctly) but then stepped out to the mrow and its parent (since it was an mrow with one child). The change to parent.isEmbellished() causes the stepping out to occur only when the outer mrow represents an embellished operator. (We only want isTop to be true in that case.)

Resolves issue #1548.

[zorkow]

lgtm.