Draft: Add option to escape LaTeX Control characters to PDF, PS and EPS export

Fix inbox#4873.

To be merged after 1.1 (introduces new strings)

This merge request adds an option to the PDF/EPS/PS export window "Escape LaTeX control characters", by default true.

drawing.svg

The option supports modern flowtexts, normal texts and SVG1.2 flowtexts (legacy).

Implementation notes:

• In the background, the implementation can escape any subset of the escape characters; the corresponding ui implementation is given in the first differing commit of this branch.
• The previous regex-based implementation is not usable for this purpose, since replacing { and \ subsequently (in any order) doesn't work as { is replaced by \{ and \ by \textbackslash{}.
• I tried to use std::string as much as possible (the wiki says to use Glib::ustring for filenames and std::string for everything else).

Known issues:

• Ideally, it should only be displayed if the Latex export radio button is checked, but afaik inx doesn't allow this at the moment.