Add PopoverWrapper and enhance DropDownListExample

Refactored `DropDownListExample` for modularity and lifecycle management. Introduced `PopoverWrapper<TView>` to enable any `View` to function as a popover, along with `ViewPopoverExtensions` for a fluent API. Added `PopoverWrapperExample` project to demonstrate usage with examples like `ListView`, forms, and `ColorPicker`.

Enhanced `Shortcut` class with a configurable `MarginThickness` property. Updated `PopoverBaseImpl` to redraw UI on visibility changes. Added comprehensive unit tests for `PopoverWrapper` and extensions. Updated `Terminal.sln` to include the new project. Added detailed documentation in `README.md`.

Improved code maintainability, modularity, and user experience.

Tests/UnitTestsParallelizable/Application/PopoverWrapperTests.cs

@@ -0,0 +1,164 @@
+using Terminal.Gui.App;
+using Terminal.Gui.Views;
+
+namespace ApplicationTests;
+
+public class PopoverWrapperTests
+{
+    [Fact]
+    public void Constructor_SetsDefaults ()
+    {
+        var wrapper = new PopoverWrapper<View> { WrappedView = new View () };
+
+        Assert.Equal ("popoverWrapper", wrapper.Id);
+        Assert.True (wrapper.CanFocus);
+        Assert.Equal (Dim.Fill (), wrapper.Width);
+        Assert.Equal (Dim.Fill (), wrapper.Height);
+        Assert.True (wrapper.ViewportSettings.HasFlag (ViewportSettingsFlags.Transparent));
+        Assert.True (wrapper.ViewportSettings.HasFlag (ViewportSettingsFlags.TransparentMouse));
+    }
+
+    [Fact]
+    public void WrappedView_CanBeSet ()
+    {
+        var view = new View { Id = "testView" };
+        var wrapper = new PopoverWrapper<View> { WrappedView = view };
+
+        Assert.Same (view, wrapper.WrappedView);
+        Assert.Equal ("testView", wrapper.WrappedView.Id);
+    }
+
+    [Fact]
+    public void EndInit_AddsWrappedViewAsSubview ()
+    {
+        var view = new View { Id = "wrapped" };
+        var wrapper = new PopoverWrapper<View> { WrappedView = view };
+
+        wrapper.BeginInit ();
+        wrapper.EndInit ();
+
+        Assert.Contains (view, wrapper.SubViews);
+        Assert.Same (wrapper, view.SuperView);
+    }
+
+    [Fact]
+    public void CanBeRegisteredAndShown ()
+    {
+        var view = new View
+        {
+            X = Pos.Center (),
+            Y = Pos.Center (),
+            Width = 20,
+            Height = 10
+        };
+
+        var wrapper = new PopoverWrapper<View> { WrappedView = view };
+        var popoverManager = new ApplicationPopover ();
+
+        popoverManager.Register (wrapper);
+        Assert.Contains (wrapper, popoverManager.Popovers);
+
+        popoverManager.Show (wrapper);
+        Assert.Equal (wrapper, popoverManager.GetActivePopover ());
+        Assert.True (wrapper.Visible);
+    }
+
+    [Fact]
+    public void QuitCommand_HidesPopover ()
+    {
+        var view = new View ();
+        var wrapper = new PopoverWrapper<View> { WrappedView = view };
+        var popoverManager = new ApplicationPopover ();
+
+        popoverManager.Register (wrapper);
+        popoverManager.Show (wrapper);
+
+        Assert.True (wrapper.Visible);
+
+        wrapper.InvokeCommand (Command.Quit);
+
+        Assert.False (wrapper.Visible);
+    }
+
+    [Fact]
+    public void AsPopover_Extension_CreatesWrapper ()
+    {
+        var view = new View { Id = "testView" };
+
+        PopoverWrapper<View> wrapper = view.AsPopover ();
+
+        Assert.NotNull (wrapper);
+        Assert.Same (view, wrapper.WrappedView);
+    }
+
+    [Fact]
+    public void AsPopover_Extension_ThrowsIfViewIsNull ()
+    {
+        View? view = null;
+
+        Assert.Throws<ArgumentNullException> (() => view!.AsPopover ());
+    }
+
+    [Fact]
+    public void WrappedView_ReceivesInput ()
+    {
+        var textField = new TextField { Width = 20 };
+        var wrapper = new PopoverWrapper<TextField> { WrappedView = textField };
+
+        wrapper.BeginInit ();
+        wrapper.EndInit ();
+
+        var popoverManager = new ApplicationPopover ();
+        popoverManager.Register (wrapper);
+        popoverManager.Show (wrapper);
+
+        Assert.True (wrapper.Visible);
+        Assert.Contains (textField, wrapper.SubViews);
+    }
+
+    [Fact]
+    public void Multiple_Types_CanBeWrapped ()
+    {
+        var label = new Label { Text = "Test" };
+        var labelWrapper = new PopoverWrapper<Label> { WrappedView = label };
+
+        var button = new Button { Title = "Click" };
+        var buttonWrapper = new PopoverWrapper<Button> { WrappedView = button };
+
+        var listView = new ListView ();
+        var listViewWrapper = new PopoverWrapper<ListView> { WrappedView = listView };
+
+        Assert.Same (label, labelWrapper.WrappedView);
+        Assert.Same (button, buttonWrapper.WrappedView);
+        Assert.Same (listView, listViewWrapper.WrappedView);
+    }
+
+    [Fact]
+    public void Current_Property_CanBeSetAndGet ()
+    {
+        var view = new View ();
+        var wrapper = new PopoverWrapper<View> { WrappedView = view };
+        var runnable = new Runnable ();
+
+        wrapper.Current = runnable;
+
+        Assert.Same (runnable, wrapper.Current);
+    }
+
+    [Fact]
+    public void Disposed_Wrapper_DisposesWrappedView ()
+    {
+        var view = new View ();
+        var wrapper = new PopoverWrapper<View> { WrappedView = view };
+
+        wrapper.BeginInit ();
+        wrapper.EndInit ();
+
+        bool viewDisposed = false;
+        view.Disposing += (s, e) => viewDisposed = true;
+
+        wrapper.Dispose ();
+
+        Assert.True (viewDisposed);
+    }
+}