Introduce the latexpar separator.

Introduce the latexpar separator.

This is the same as the parbreak separator and is represented on screen
as the old parbreak. Old parbreak separators are converted to latexpar
separators when they are used for introducing blank lines in the
latex output rather than for separating environments.
Instead, parbreak separators are now represented on screen by a
double line. In essence, latexpar and parbreak separators produce
the same output but are represented differently on screen.
The context menu does not account for latexpar separators and only
"true" separators can be turned each into the other one.