Refactor document builder

[vkbo]

Summary:

This PR refactors the NWBuildDocument class by making the format classes more uniform,a nd thus simplify the loops calling them. It also adds a ToRaw class for writing the dump of the minimally processed text with novelWriter format.

This is a follow-up of MR #2045

Related Issue(s):

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