Enable/disable the add/remove model buttons at the right times

[robertbrignull]

Hooks up the disabled attribute for the add/remove model buttons in the model editor.

Uses the same logic as the method modeling panel, and pulls out the implementation to a shared location. Although it turns out the model editor doesn't need the logic for disabling the remove button, because it's never shown in situations when it would be disabled.

Also adds some tests that the buttons are shown / enabled at the right times.

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.

[robertbrignull]

I had to change around where the label on the button is, taking inspiration from

vscode-codeql/extensions/ql-vscode/src/view/method-modeling/__tests__/MultipleModeledMethodsPanel.spec.tsx

Line 50 in 8ecc31f

screen.getByLabelText("Next modeling").getElementsByTagName("input")[0],