Fix problem with branch handling. The problem was that we were not

Fix problem with branch handling. The problem was that we were not
dealing properly with the paragraph separator tag.

We really need to use that tag as a kind of general marker for which
tags we're responsible for in a given paragraph and which tags we are
not. So the changes to InsetText.cpp use the tag as that kind of marker.

Note that, as of this commit, the User Guide again exports without any
kind of error. I haven't yet checked the other manuals.

This fixes bug #8022.