enhance: add Layout prop for ButtonGroup

src/Shared/HandyControl_Shared/Controls/Button/ButtonGroup.cs

@@ -1,12 +1,14 @@
 using System.Windows;
 using System.Windows.Controls;
 using System.Windows.Controls.Primitives;
+using System.Windows.Media;
+using HandyControl.Data;
 
 namespace HandyControl.Controls
 {
     public class ButtonGroup : ItemsControl
     {
-        protected override bool IsItemItsOwnContainerOverride(object item) => item is Button || item is RadioButton || item is ToggleButton;
+        protected override bool IsItemItsOwnContainerOverride(object item) => item is Button or RadioButton or ToggleButton;
 
         public static readonly DependencyProperty OrientationProperty = DependencyProperty.Register(
             "Orientation", typeof(Orientation), typeof(ButtonGroup), new PropertyMetadata(default(Orientation)));
@@ -17,10 +19,17 @@ namespace HandyControl.Controls
             set => SetValue(OrientationProperty, value);
         }
 
-        protected override void OnVisualChildrenChanged(DependencyObject visualAdded, DependencyObject visualRemoved)
-        {
-            base.OnVisualChildrenChanged(visualAdded, visualRemoved);
+        public static readonly DependencyProperty LayoutProperty = DependencyProperty.Register(
+            "Layout", typeof(LinearLayout), typeof(ButtonGroup), new PropertyMetadata(LinearLayout.Uniform));
 
+        public LinearLayout Layout
+        {
+            get => (LinearLayout) GetValue(LayoutProperty);
+            set => SetValue(LayoutProperty, value);
+        }
+
+        protected override void OnRender(DrawingContext drawingContext)
+        {
             var count = Items.Count;
             for (var i = 0; i < count; i++)
             {