Various GUI improvements

[vkbo]

Summary:

This PR:

• Adds an edit button to the viewer, which will open the document in the editor.
• Fixes an issue where the app could no longer be closed if the project was closed first.
• Fixes an issue with saving the Outline view details panel splitter position if the Outline tab has not been activated during a writing session.

Related Issue(s):

Closes #1963

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