Sanitize InsetMathHull and add a check for mutability in LFUN_MATH_MUTATE

Sanitize InsetMathHull and add a check for mutability in LFUN_MATH_MUTATE

Remove in particular all comparisons < and >= involving HullType.

Add a guard to make sure that mutate() only operates on types it has been
designed for. Then I figured I could use this new knowledge to give feedback
when math-mutate is not implemented via getStatus(). (To test this, insert a
regexp in Advanced Search & Replace and try to change it into a standard
equation via the contextual menu.)