LaTeX writer: Protect graphics in headers.

Graphics in `\section`/`\subsection` etc titles need to be `\protect`ed.

This adds a state value and manually turns it on before every invocation
of `sectionHeader` and manually turns it off after. Using a writer value
and applying `local` would probably be cleaner, but this fits with the
current style.
LaTeX writer: Test for protecting images in header.