GUI: Basic GUI elements, DataWidget pages

[tangkong]

Description

• Adds DataWidget and NameDescTagsWidget, components for making widgets that communicate with dataclasses
• Adds EntryItem and RootTree for representing Entry dataclasses in a tree view
• Adds one sample Collection widget to demonstrate how to use these widget mixins

To-Do, possibly in followup issues:

• Make a QDataclassBridge type for a set specifically, to support tag addition
• Rework Tags entry when we figure out where we're storing tags

Motivation and Context

We need a framework to make a GUI.

I lifted most of this from ATEF, and as such we should be free to challenge any of the things we've built here. We've found it quite useful, and I think it can be similarly useful here. There should definitely be a keen eye taken to these. While we've proven they work, they shouldn't be taken as gospel. I say this partially for the review of this PR, but maybe mostly for future efforts that use these components.

The RootModel is very incomplete, and client interactions still need to be fleshed out. I can see a world where we create the client on GUI startup and have some helper for fetching it, rather than passing it to every widget instance. There was also an intent to make the data model loading lazy, and the RootModel is where this would manifest. But this PR is already long enough.

If I ever have to vendor the DataWidget / QDataclassBridge stuff again I'm going to upstream it into pcdsutils.

How Has This Been Tested?

Added some tests.

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

[shilorigins]

I'm happy with this, thanks!