Fixes #2114. 3D Effect

[tig]

Fixes

• Norton Commander style buttons: Add half-height bottom shadow for Effect3D #2144

Todo

• Extend Margin to have a "Shadow" Style - When enabled draws the correct half-height block glyphs in the right places
• Extend HighlightStyle to support shadow animation on Pressed
• Enable Button to use Margin.EnableShadow
• Figure out how to make Button with shadow work reliably and automatically.
• Test & Enable EnableShadow on other Views where it may make sense visually.

Background

When I created Frames (now Adornments) i had a vision that Margin could be used to enable the 3D effect that we had mostly working in v1, but got negated with all the changes to v2.

I had a shower thought recently on this and decided to quickly code a prototype up to see if my instincts were right.

This PR demonstrates that I was. The concept is leveraging Margin like this:

 var button = new Button
 {
     X = Pos.Center (),
     Y = Pos.Center (), Text = "Press me!"
 };
 button.Margin.Thickness = new Thickness (0, 0, 1, 1);
 button.Margin.Add (
                    new View ()
                    {
                        X = Pos.AnchorEnd (1),
                        Y = 1,
                        Width = 1,
                        Height = Dim.Fill (),
                        ColorScheme = new ColorScheme (new Attribute (ColorName.DarkGray))
                    },
                    new View ()
                    {
                        X = 1,
                        Y = Pos.AnchorEnd (1),
                        Width = Dim.Fill(),
                        Height = 1,
                        ColorScheme = new ColorScheme (new Attribute (ColorName.DarkGray))
                    }
                    );

 bool pressed = false;
 button.Accept += (s, e) =>
                  {
                      pressed = !pressed;

                      if (pressed)
                      {
                          button.Margin.Thickness = new Thickness (1, 1, 0, 0);
                      }
                      else
                      {
                          button.Margin.Thickness = new Thickness (0, 0, 1, 1);
                      }
                  };

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]

Latest:

It's this easy:

        var button = new Button
        {
            X = Pos.Center (),
            Y = Pos.Center (), Text = "Press me!",
        };
        button.Margin.Shadow = true;

Of course, the animation will only work on WindowsDriver, because it's the only one that supports distinct mouse up/down events.

[tig]

Latest

• Enables Window.DefaultShadow and Button.DefaultShadow to be configured via Config Manager.
• The UI Catalog theme sets both to true.
• AdornmentsEditor now has a Shadow checkbox

(Since the Default theme doesn't explicitly set either they stay set even after changing back to the Default theme).