Add new usage for the TSF export plugin. Usable with the menu
"Extension/Fablab/Exporter en fichier Trotec Spool File (TSF)". You can
the specify the trotec spool directory and then generate your tsf files
directly on the right place. This way you can also export only selected
element and not the entire page.

Correct bugs on windows when calling convert (imagemagick) to generate
engraving bmp.