Tests for special symbols lists

[bertfrees]

This is a new feature proposed by @EdmarS. Dedicon's current braille converter generates a list with braille symbols that need some explanation. For example some symbols have a special meaning when used in the in the context of mathematics. The list is generated at the beginning of the document (not at the beginning of each volume) and it should only contain symbols that are actually used in the book.

Related:

• Tests for generated content #33

[bertfrees]

In order to support this feature in a generic way I think considerable design changes are needed. However depending on how widely this feature will be used it may be acceptable to implement it in an ad hoc way (for example Dedicon could just rewrite their current solution).