Added LayoutUpdated event.

Fixes #825.

src/Avalonia.Layout/Layoutable.cs

@@ -154,6 +154,11 @@ namespace Avalonia.Layout
                 VerticalAlignmentProperty);
         }
 
+        /// <summary>
+        /// Occurs when a layout pass completes for the control.
+        /// </summary>
+        public event EventHandler LayoutUpdated;
+
         /// <summary>
         /// Gets or sets the width of the element.
         /// </summary>
@@ -357,6 +362,8 @@ namespace Avalonia.Layout
                 IsArrangeValid = true;
                 ArrangeCore(rect);
                 _previousArrange = rect;
+
+                LayoutUpdated?.Invoke(this, EventArgs.Empty);
             }
         }