This done according to the TeXbook. This class replaces the individual
isMathXXX() methods. The mathClass() method (currently unused) is
provided for the following insets:
* InsetMathChar (with a revised list of affected characters)
* InsetMathSymbol: the class is given by the `extra' field
Operators defined in lib/symbols (e.g. \log) are MC_OP
* InsetMathFrac is MC_INNER (except nicefrac and units)
* InsetDelimiters is MC_INNER
* InsetStackrel is MC_REL
* The class of InsetScript is the class of the last element of its
nucleus (yes, it is a hack, but doing it right is more work).
Remove the explicit spacing that was done in the different insets. The spacing
will be reintroduced properly in a forthcoming commit.
