Introduce hooks to encapsulate macro code of MathRow

Introduce hooks to encapsulate macro code of MathRow

New hooks (before|after)(Metrics|Draw) are run by the MathRow code
as needed. They are actually also used internally by the MathMacro
code.

The gain is that most traces of explicit macro handling (and
the MathRow::Element::macro member) have been removed from MathRw.cpp.

The next step will be to extend the tokenizing process to allow for
linearizing insets other than macros.