replace v6 with experimental v7 code

src/MpvNet.Windows/WPF/HandyControl/Controls/SimplePanel.cs

@@ -0,0 +1,37 @@
+
+
+using System.Windows;
+using System.Windows.Controls;
+
+namespace HandyControl.Controls
+{
+    public class SimplePanel : Panel
+    {
+        protected override Size MeasureOverride(Size constraint)
+        {
+            var maxSize = new Size();
+
+            foreach (UIElement child in InternalChildren)
+            {
+                if (child != null)
+                {
+                    child.Measure(constraint);
+                    maxSize.Width = Math.Max(maxSize.Width, child.DesiredSize.Width);
+                    maxSize.Height = Math.Max(maxSize.Height, child.DesiredSize.Height);
+                }
+            }
+
+            return maxSize;
+        }
+
+        protected override Size ArrangeOverride(Size arrangeSize)
+        {
+            foreach (UIElement child in InternalChildren)
+            {
+                child?.Arrange(new Rect(arrangeSize));
+            }
+
+            return arrangeSize;
+        }
+    }
+}