Add a QTextDocument format class for previews

[vkbo]

Summary:

This PR adds a new class based on the Tokenizer that generates a QTextDocument directly without going via the HTML generator. This gives a lot more control of the format as we are no longer restricted to the limited HTML4 subset implemented by Qt a very long time ago.

This is also intended to be used for the Manuscript build preview, but since the document cannot be serialised to a cache, it is not a straight drop-in replacement for the HTML. The class may need a pack/unpack method for that, which can probably be written from the token array.

Related Issue(s):

Closes #1882

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