MNT: move context menu functionality upstream into RootTreeView

[tangkong]

Description

• Adds common RootTreeView which holds the RootTree model
  • Upstreams standard settings and context menu support
• Fills the top-level entry whenever RootTree model is created, to ensure at least first level of children display

Motivation and Context

Closes #101

How Has This Been Tested?

Interactively, and some tests have been added

Now you can double-click-to-open and right-click inside of other pages that have Tree Views
Now all pages with tree views will open with at least the first set of children populated.

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]

Looks good to me, thanks!