Implement new Dim/Pos capabilities, referencing the elements of another view

Example/demo.cs

@@ -92,12 +92,33 @@ static class Demo {
 
 		Application.MainLoop.AddTimeout (TimeSpan.FromMilliseconds (300), timer);
 
+
+		// A little convoluted, this is because I am using this to test the
+		// layout based on referencing elements of another view:
+
+		var login = new Label ("Login: ") { X = 3, Y = 6 };
+		var password = new Label ("Password: ") { 
+			X = Pos.Left (login), 
+			Y = Pos.Bottom (login) + 1 
+		};
+		var loginText = new TextField ("") { 
+			X = Pos.Right (password),  
+			Y = Pos.Top (login), 
+			Width = 40 
+		};
+		var passText = new TextField ("") {
+			Secret = true,
+			X = Pos.Left (loginText),
+			Y = Pos.Top (password),
+			Width = Dim.Width (loginText)
+		};
+
 		// Add some content
 		container.Add (
-			new Label (3, 6, "Login: "),
-			new TextField (14, 6, 40, ""),
-			new Label (3, 8, "Password: "),
-			new TextField (14, 8, 40, "") { Secret = true },
+			login,
+			loginText,
+			password,
+			passText,
 			new FrameView (new Rect (3, 10, 25, 6), "Options"){
 				new CheckBox (1, 0, "Remember me"),
 				new RadioGroup (1, 2, new [] { "_Personal", "_Company" }),