Cleanup of spacing in mathed

Cleanup of spacing in mathed

This is a first cleanup step. More complex rules have to be
implemented on top of this.

Use proper spacing \thinmuskip, \medmuskip and \thickmuskip instead of
ad-hoc values.

Rename isRelOp to isMathRel and introduce isMathBin and isMathPunct
(for InsetMathChar and InsetMathSymbol). Update the categories of
characters in InsetMathChar according to LaTeX source (fontmath.ltx).

Set correctly the spacing around mathrel, mathbin and mathpunct
elements. Use \thinmuskip around MathDelim instead of a hardcoded 4.

This is related to bug #8883.