Make the editor actually Plain Text

[vkbo]

Summary:

This PR makes the text editor in novelWriter actually plain text. So far, it's been a rich text editor with rich text editing disabled. At some point, there was a reason for this. There are still some drawbacks, but the benefits outweigh them.

Pros

• It loads a lot faster since it doesn't have to take into account special text content like lists, images and tables. Which we don't support anyway.
• The syntax highlighter works a lot more smoothly since it is designed and optimised for the plain text view.
• Re-running the highlighter is no longer a performance bottleneck, so we can now rerun it for spell checking purposes even for large documents.
• Scrolling past the end of the document is supported out of the box.
• Centring the cursor when skipping to a specific line is natively supported.
• There is no longer a need for the big doc limit setting, and no need to cap the max size of a text document. Performance when loading will suffer when a document reaches the 10MB range, but the editor still works, and works smoothly once the document is loaded.

Cons

• The document now scrolls a single line at a time, so there is no smooth scrolling anymore.
• The feature to keep the current line fixed is not as smooth anymore due to the above reason, but it does still work.

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