Improve editor stats handling and test coverage

[vkbo]

Summary:

This PR makes a few improvements to how the editor processes changes of the document content. The percentage counter in the footer is now showing cursor position percentage, not line percentage, and the logic of how these stats are processed internally has been refactored. Test coverage has also been improved on this and the search functionality.

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