make library collection configurable and persistent

[peterstadler]

this is a follow up to #37 and #49: some user data may be stored in the library collection, especially controlled vocabulary and project specific lists of persons, bibliographic references, etc.

None(?) of these files are created/modified with the MerMEId editor, yet they are necessary for the MerMEId to work and they are likely to be customized on the project level. So the approach might be slightly different to our $data-root. Instead of copying/moving the library collection outside of the application collection, we might look for files within an customizable $library-root and only fall back to the files within the library collection of the MerMEId installation when no files are found.

[claudius108]

I would vote for having the user data in a Git repository.

One can use the Tuttle EXPath app, which adds version control to data collections in eXist-db applications.

Check the Tuttle EXPath app, with a description at https://faq.teipublisher.com/annotations/git/tuttle/. The GitHub repo seems to be at https://github.com/eeditiones/tuttle, and the documentation at https://eeditiones.github.io/tuttle-doc/.