Fix bugs in the document viewer

[vkbo]

Summary:

This PR fixes a number of bugs with the new document viewer in 2.5:

• Tab stop distances are now properly set after a document is loaded. Loading a new document resets it to default value, and need to be updated.
• The Tokenizer can now properly handle multiple formatting markers that are set on the same character location. Previously, the order of the position-sorted markers would affect whether they came out correctly or not.
• The Tokenizer now only applies dialogue highlighting to plain text paragraphs in novel documents. Previously, the viewer would highlight all quotes as dialog.

Related Issue(s):

Closes #1996
Closes #2011
Closes #2012
Closes #2013

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