feat: add control.

src/Ursa/Controls/BackTop/BackTop.cs

@@ -5,7 +5,7 @@ using Avalonia.Media;
 
 namespace Ursa.Controls.BackTop;
 
-public class BackTop: Control
+public class BackTop : Control
 {
     public static readonly AttachedProperty<bool> AttachProperty =
         AvaloniaProperty.RegisterAttached<BackTop, Control, bool>("Attach");

src/Ursa/Controls/BackTop/BackTopButton.cs

@@ -0,0 +1,22 @@
+using Avalonia;
+using Avalonia.Controls;
+
+namespace Ursa.Controls;
+
+public class BackTopButton: Button
+{
+    public static readonly StyledProperty<Control> TargetProperty = AvaloniaProperty.Register<BackTopButton, Control>(
+        nameof(Target));
+
+    public Control Target
+    {
+        get => GetValue(TargetProperty);
+        set => SetValue(TargetProperty, value);
+    }
+    
+    protected override void OnClick()
+    {
+        base.OnClick();
+        
+    }
+}