Add CSV export for Outline View

[vkbo]

Summary:

This PR adds an "Export CSV" button to the toolbar of the Project Outline. It does what it says it does, exports the content of the outline to a CSV file that can be opened as a spreadsheet or table in a compatible app. The content exported should be exactly the same as visible in the table, including header names, column order, and ignoring hidden columns.

Related Issue(s):

Closes #1507

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