Close key on window can be null - needed for turning off closing by key

Author: dima.am

src/Myra/Graphics2D/UI/Misc/Window.cs

@@ -144,7 +144,7 @@ public override VerticalAlignment VerticalAlignment
 
 		[Category("Behavior")]
 		[DefaultValue(Keys.Escape)]
-		public Keys CloseKey { get; set; }
+		public Keys? CloseKey { get; set; }
 
 		private bool IsWindowPlaced { get; set; }