Strike out (in the output) deleted display math with track-changes

Strike out (in the output) deleted display math with track-changes

Showing deleted display math by enabling "Show Changes in Output" was
only possible with dvi (through dvipost). Although LyX strikes out
such formulas on screen, it was impossible obtaining an output
directly using pdflatex (or other engines producing pdf) because
ulem cannot cope with display math material and gives errors.
The solution is to strike out by ourselves such deleted formulas.
I took into account several options. One of them would produce
an output similar to dvipost (which strikes out each element), but
would have required much more changes in the output routines.
Eventually, I opted for using tikz, which gives a more clean
output (as it requires to simply adding a preamble and a postamble
to the latex code of any displayed math, instead of a mark up
tailored to each particular math construct). The look of the pdf
output is similar to the way LyX strikes out the equations on screen.

Fixes #9678