Add a panel background surface

[Aaronontheweb]

Summary

• Add an optional PanelNode background surface.
• Preserve the selected background after child color resets.
• Approve the additive public API.

Validation

• All 1,713 tests passed.
• The focused panel and API tests passed.
• Slopwatch found no issue in the changed source and test files.
• git diff --check passed.

Closes #378