GUI: add top level tree to main window

[tangkong]

Description

• Adds top-level tree to main window
• Adds icons to the TreeModel
• Adds fill_uuid method to TreeModel, which grabs data from client if possible
• Adds context menu on tree view for accessing custom actions. Currently only has an open-page action, but later could include more
• Adds a couple tests

Motivation and Context

closes #44

How Has This Been Tested?

Interactively. Am I getting lazy with gui tests? Do I just fear them? Is it a superposition? 🤔

I added a couple tests, gui testing can be finicky

Where Has This Been Documented?

This PR.

Pre-merge checklist

• Code works interactively
• Code follows the style guide
• Code contains descriptive docstrings, including context and API
• New/changed functions and methods are covered in the test suite where possible
• Test suite passes locally
• Test suite passes on GitHub Actions
• Ran docs/pre-release-notes.sh and created a pre-release documentation page
• Pre-release docs include context, functional descriptions, and contributors as appropriate

[ZLLentz]

I'm 👍 on the changes here, again I don't have much to suggest.

[tangkong]

I'll merge this tomorrow morning, if there are no further comments or reviews.