Cleanup MathMLStream

Cleanup MathMLStream

This is preparatory work for fixing ticket #13069.

Remove direct accesses to the underlying stream of MathMLStream in
InsetMathChar, InsetMathSpecialChar, and in all << operators other
than MathMLStream << docstring. This will allow to add a hook later in
this operator.

Move default values of MathMLStream private members to their definition.

Get rid of line_ member, which is not used.