Introduce the notion of math class

Introduce the notion of math class

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.