Set e.Handled on command execution.

Otherwise the `Click` event bubbles upwards causing #566.

Fixes #566

src/Avalonia.Controls/Button.cs

@@ -207,7 +207,11 @@ namespace Avalonia.Controls
         /// <param name="e">The event args.</param>
         protected virtual void OnClick(RoutedEventArgs e)
         {
-            Command?.Execute(CommandParameter);
+            if (Command != null)
+            {
+                Command.Execute(CommandParameter);
+                e.Handled = true;
+            }
         }
 
         /// <inheritdoc/>

src/Avalonia.Controls/MenuItem.cs

@@ -102,6 +102,11 @@ namespace Avalonia.Controls
             AccessKeyHandler.AccessKeyPressedEvent.AddClassHandler<MenuItem>(x => x.AccessKeyPressed);
         }
 
+        public MenuItem()
+        {
+
+        }
+
         /// <summary>
         /// Occurs when a <see cref="MenuItem"/> without a submenu is clicked.
         /// </summary>
@@ -192,7 +197,11 @@ namespace Avalonia.Controls
         /// <param name="e">The click event args.</param>
         protected virtual void OnClick(RoutedEventArgs e)
         {
-            Command?.Execute(CommandParameter);
+            if (Command != null)
+            {
+                Command.Execute(CommandParameter);
+                e.Handled = true;
+            }
         }
 
         /// <summary>

src/Avalonia.Styling/Styling/Style.cs

@@ -61,12 +61,12 @@ namespace Avalonia.Styling
         }
 
         /// <summary>
-        /// Gets or sets style's selector.
+        /// Gets or sets the style's selector.
         /// </summary>
         public Selector Selector { get; set; }
 
         /// <summary>
-        /// Gets or sets style's setters.
+        /// Gets or sets the style's setters.
         /// </summary>
         [Content]
         public IEnumerable<ISetter> Setters { get; set; } = new List<ISetter>();