Improve spacing of BOX elements

Improve spacing of BOX elements

Tweak the algorithm so that a BOX math row element can have some
spacing. To this end, MathRow::before/after do not look at the type of
an element for deciding when to skip it, but rather to its math class.

In the new setting, the spacing algorithm works on all elements, but
skips the MC_UNKNOWN elements as if they were not present. As a
consequence, the two element types BEGIN and END have been replaced by
a single DUMMY (they can be recognized from their class).

To simply the code, add a new `mclass' argument to the
MathRow::Element constructor (default is MC_UNKNOWN).