Add a tool to download spell check dictionaries, alternative 2

[vkbo]

Summary:

This PR adds an "Add Dictionaries" dialog that is available on Windows, or when novelWriter is running in DEBUG mode. The tool will process LibreOffice Extension files (*.oxt) and Free Office Dictionary files (*.sox) and extract the *.aff and *.dic Hunspell files contained in them, if any exist. These will be copied to the correct location so the spell checker library can pick them up.

This is an alternative implementation to PR #1610

Related Issue(s):

Closes #982

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