Add a document outline dropdown

[vkbo]

Summary:

This PR adds a dropdown menu in the header of both the document editor and viewer for quick navigation to any heading defined in it. The PR also cleans refactor a lot of the calls from the editor and viewer to other parts of the GUI to use signals and slots instead of direct calls.

Related Issue(s):

Closes #1059

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