feat: support dialog resize.

src/Ursa/Controls/Resizers/DialogResizer.cs

@@ -0,0 +1,19 @@
+using Avalonia;
+using Avalonia.Controls.Primitives;
+
+namespace Ursa.Controls;
+
+public class DialogResizer: TemplatedControl
+{
+    public static readonly StyledProperty<ResizeDirection> ResizeDirectionProperty = AvaloniaProperty.Register<DialogResizer, ResizeDirection>(
+        nameof(ResizeDirection));
+
+    /// <summary>
+    /// Defines what direction the dialog is allowed to be resized.
+    /// </summary>
+    public ResizeDirection ResizeDirection
+    {
+        get => GetValue(ResizeDirectionProperty);
+        set => SetValue(ResizeDirectionProperty, value);
+    }
+}