Reimplement support for numbering of equation previews

Reimplement support for numbering of equation previews

This patch simplifies and fixes the generation of previews with
numbering. Instead of saving the counter values (which is complicated
and error prone), the \thequation macro is set to the numbers_ string
which has been computed by updateBuffer for screen display.

Harcoding the number in the formula means that two equations with
different numbers have different previews, which is the subject of the
bug.

The special code for saving counters in updateBuffer and outputing
them in preparePreview from 645f9fdf (ticket #6894) is removed and
replaced by this simpler approach.

This fixes ticket #11535.