Drop saving fullscreen flag

[vkbo]

Summary:

This PR removes the caching of the full screen state to the config file. This never worked properly anyway since the full screen flag internally in Qt doesn't necessarily match the window manager.

This PR also adds annotations to the main GUI class.

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