Add import and export to project word list

[vkbo]

Summary:

This PR adds import and export to the Project Word List tool. The import is quite robust, and you can even feed it an entire novel as long as the file is plain text. It will add every unique word in it to the list. This is of course not advisable, but it's a good stress test. The export writes a text file with one word per line.

Related Issue(s):

Closes #1560

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed