MathMathML

The LaTeXML class contains some code that is
not specific to LaTeXML but to MathML.
This code can be shared between LaTeXML and Mathoid.
This change introduces a new class called MathMathML
and moves the shared code from the LaTeXML class to this
new class.

Bug: 65973
Change-Id: I50517ba83f9a0d2aa8e237f062f18e4319ddbac8