Update subview handling and use C# 12 collection exprs

Refactor subview checks to use GetSubViews with padding, ensuring accurate layout calculations. Update button list initialization in MessageBoxes.cs to use the C# 12 collection expression for conciseness. Minor formatting cleanup in Dialog.cs. No functional changes to dialog padding logic.

Examples/UICatalog/Scenarios/MessageBoxes.cs

@@ -240,7 +240,7 @@ public class MessageBoxes : Scenario
                                                int numButtons = int.Parse (numButtonsEdit.Text);
                                                int defaultButton = int.Parse (defaultButtonEdit.Text);
 
-                                               List<string> btns = new ();
+                                               List<string> btns = [];
 
                                                for (var i = 0; i < numButtons; i++)
                                                {