Improve document editing

[vkbo]

Summary:

This PR includes various improvements to how documents are handled in the editor and main GUI:

• The editor now has a signal emitted when documents are closed and opened.
• The novel view now listens to this signal when highlighting the current document.
• This feature has also been added to the project tree,
• The opening and closing procedure for editor documents have been refactored a little.
• The editor now uses the PyQt6 compliant wrapper for passing signals to non-slot functions.

Related Issue(s):

Closes #1981

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