Native MathML: add a workaround for <mlabeledtr>, rowspacing/columnspacing

Created by: fred-wang

is used in the code generated by MathJax from LaTeX (for equation labeling) but browsers like Webkit, Gecko or Opera do not support this element:

https://bugzilla.mozilla.org/show_bug.cgi?id=689641 https://bugs.webkit.org/show_bug.cgi?id=85732 http://www.w3.org/TR/mathml-for-css/#d2e2181

cf the discussion on mathjax-dev