Fixes #2944. TreeView unit tests fail intermittently: Ensures Attribute and Color are read only value types

[tig]

Fixes

• Fixes TreeView sometimes causing unit test failure #2944

Todo:

• Make Color a readonly value type with unit test to prove it
  • Change Color.ColorName to be get only. We're lucky in that the only code in the project that does someAttribute.ColorName = colorName are in unit tests! To convince myself I'm correct on this, I commented out all those tests that set ColorName and the TreeView case no longer fails.
  • Change Color.R/G/B/A/Rgba/ to be get only.
• Fix things that the above broke
• Beef up unit tests

For another time:

• Change ColorScheme in the same way. I spent an hour yesterday on an issue where I had done var cs = colorScheme vs var cs = new ColorScheme(colorScheme). This one is gonna be harder, as there's tons of code that will break, I fear. We'll do this in : Colors: ColorSchemes refactor - Should honor terminal color settings etc... #2381

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

[tig]

On this:

• Change Attribute.Default to be a static dynamic property with only a geter.

Now that both Color and Attribute are value types, this is no longer needed, so I took it back to the way it was before as it's slightly more performant...

[JsonConverter (typeof (AttributeJsonConverter))]
public readonly struct Attribute : IEquatable<Attribute> {
	/// <summary>
	/// Default empty attribute.
	/// </summary>
	/// </remarks>
	public static readonly Attribute Default = new (Color.White, Color.Black);

[BDisp]

Veryyyyy nice work. I love it.