feat:Add several predefined SizeAnimations.

demo/Ursa.Demo/Pages/NavMenuDemo.axaml

@@ -41,9 +41,7 @@
                 ItemsSource="{Binding MenuItems}"
                 SelectedItem="{Binding SelectedMenuItem}"
                 SubMenuBinding="{Binding Children}"
-                u:SizeAnimationHelper.TriggerAvaloniaProperty="{x:Static u:NavMenu.IsHorizontalCollapsedProperty}"
-                u:SizeAnimationHelper.CreateAnimation="{x:Static views:NavMenuDemo.NavMenuAnimation}"
-                u:SizeAnimationHelper.EnableWHAnimation="True">
+                Classes="enable_animation">
                 <u:NavMenu.Styles>
                     <Style x:DataType="vm:MenuItem" Selector="u|NavMenuItem">
                         <Setter Property="IsSeparator" Value="{Binding IsSeparator}" />

demo/Ursa.Demo/Pages/NavMenuDemo.axaml.cs

@@ -17,38 +17,4 @@ public partial class NavMenuDemo : UserControl
     {
         InitializeComponent();
     }
-
-    public static SizeAnimationHelperAnimationGeneratorDelegate NavMenuAnimation { get; } =
-        (_, oldDesiredSize, newDesiredSize) =>
-        {
-            if (oldDesiredSize.Width > newDesiredSize.Width)
-                newDesiredSize = newDesiredSize.WithWidth(newDesiredSize.Width + 20);
-            return new Animation
-            {
-                Duration = TimeSpan.FromMilliseconds(300),
-                Easing = new CubicEaseInOut(),
-                FillMode = FillMode.None,
-                Children =
-                {
-                    new KeyFrame
-                    {
-                        Cue = new Cue(0.0),
-                        Setters =
-                        {
-                            new Setter(WidthProperty, oldDesiredSize.Width),
-                            new Setter(HeightProperty, oldDesiredSize.Height)
-                        }
-                    },
-                    new KeyFrame
-                    {
-                        Cue = new Cue(1.0),
-                        Setters =
-                        {
-                            new Setter(WidthProperty, newDesiredSize.Width),
-                            new Setter(HeightProperty, newDesiredSize.Height)
-                        }
-                    }
-                }
-            };
-        };
 }