[mathml_element] Use changed and child_changed virtuals.
[mathml_element] Use changed and child_changed virtuals.

wavewave/lasem,janmarthedal/mathml2svg,janmarthedal/mathml2svg,janmarthedal/mathml2svg,wavewave/lasem,wavewave/lasem,gjtorikian/lasem,gjtorikian/lasem,janmarthedal/mathml2svg,wavewave/lasem,gjtorikian/lasem,gjtorikian/lasem