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);
+    }
+}

src/Ursa/Controls/Resizers/DialogResizerThumb.cs

@@ -0,0 +1,48 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Controls.Primitives;
+using Avalonia.Input;
+using Avalonia.LogicalTree;
+using Ursa.Controls.OverlayShared;
+
+namespace Ursa.Controls;
+
+public class DialogResizerThumb: Thumb
+{
+    private OverlayFeedbackElement? _dialog;
+    
+    public static readonly StyledProperty<ResizeDirection> ResizeDirectionProperty = AvaloniaProperty.Register<DialogResizerThumb, ResizeDirection>(
+        nameof(ResizeDirection));
+
+    public ResizeDirection ResizeDirection
+    {
+        get => GetValue(ResizeDirectionProperty);
+        set => SetValue(ResizeDirectionProperty, value);
+    }
+
+    protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
+    {
+        base.OnApplyTemplate(e);
+        _dialog = this.FindLogicalAncestorOfType<OverlayFeedbackElement>();
+    }
+
+    protected override void OnPointerPressed(PointerPressedEventArgs e)
+    {
+        base.OnPointerPressed(e);
+        if (_dialog is null) return;
+        if (!e.GetCurrentPoint(this).Properties.IsLeftButtonPressed) return;
+        var windowEdge = ResizeDirection switch
+        {
+            ResizeDirection.Top => WindowEdge.North,
+            ResizeDirection.TopRight => WindowEdge.NorthEast,
+            ResizeDirection.Right => WindowEdge.East,
+            ResizeDirection.BottomRight => WindowEdge.SouthEast,
+            ResizeDirection.Bottom => WindowEdge.South,
+            ResizeDirection.BottomLeft => WindowEdge.SouthWest,
+            ResizeDirection.Left => WindowEdge.West,
+            ResizeDirection.TopLeft => WindowEdge.NorthWest,
+            _ => throw new ArgumentOutOfRangeException()
+        };
+        _dialog.BeginResizeDrag(windowEdge, e);
+    }
+}

src/Ursa/Controls/Resizers/ResizeDirection.cs

@@ -1,5 +1,6 @@
 namespace Ursa.Controls;
 
+[Flags]
 public enum ResizeDirection
 {
     Top,