IWindowImpl: IsEnabled method

TODO: macOS implementation

src/Avalonia.Controls/Platform/IWindowImpl.cs

@@ -37,6 +37,11 @@ namespace Avalonia.Platform
         /// <param name="enable">true if the window is enabled, or false if it is disabled.</param>
         void SetEnabled(bool enable);
 
+        /// <summary>
+        /// Gets a value to indicate if the window is currently enabled.
+        /// </summary>
+        bool IsEnabled { get; }
+
         /// <summary>
         /// Called when a disabled window received input. Can be used to activate child windows.
         /// </summary>

src/Avalonia.DesignerSupport/Remote/PreviewerWindowImpl.cs

@@ -91,6 +91,8 @@ namespace Avalonia.DesignerSupport.Remote
 
         public bool NeedsManagedDecorations => false;
 
+        public bool IsEnabled => true;
+
         public void Activate()
         {
         }

src/Avalonia.DesignerSupport/Remote/Stubs.cs

@@ -47,6 +47,8 @@ namespace Avalonia.DesignerSupport.Remote
 
         public Thickness OffScreenMargin { get; } = new Thickness();
 
+        public bool IsEnabled => true;
+
         public WindowStub(IWindowImpl parent = null)
         {
             if (parent != null)

src/Avalonia.Native/WindowImpl.cs

@@ -220,5 +220,8 @@ namespace Avalonia.Native
         {
             _native.SetEnabled(enable.AsComBool());
         }
+        
+        // TODO: Implement
+        public bool IsEnabled => throw new NotImplementedException();
     }
 }

src/Avalonia.X11/X11Window.cs

@@ -1041,6 +1041,8 @@ namespace Avalonia.X11
             _disabled = !enable;
         }
 
+        public bool IsEnabled => !_disabled;
+
         public void SetExtendClientAreaToDecorationsHint(bool extendIntoClientAreaHint)
         {
         }

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -595,6 +595,8 @@ namespace Avalonia.Win32
 
         public void SetEnabled(bool enable) => EnableWindow(_hwnd, enable);
 
+        public bool IsEnabled => IsWindowEnabled(_hwnd);
+
         public void BeginMoveDrag(PointerPressedEventArgs e)
         {
             _mouseDevice.Capture(null);