Improve project handling, cursor position, and Main GUI inheritance

[vkbo]

Summary:

This is a refactor/cleanup PR that restructures how various components interact with the Main GUI. Mostly through signals where possible. It also changes how document cursor position is saved. It is now saved even if the document is unchanged.

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