Tests for integrating tactile drawings

[bertfrees]

This is currently out of scope and it's also not clear how much integration is required.

For example, a simple solution would be to automatically insert placeholders in the PEF, and to manually replace them with the tactile drawings after embossing.

A more advanced solution would be to include the drawings in the PEF, either as image files (e.g. SVG) or as references to files. This only makes sense if the whole book can be printed (so no swell paper images).