add groupbox style

HandyControlDemo/UserControl/Styles/GroupBoxDemoCtl.xaml.cs

@@ -0,0 +1,10 @@
+namespace HandyControlDemo.UserControl
+{
+    public partial class GroupBoxDemoCtl
+    {
+        public GroupBoxDemoCtl()
+        {
+            InitializeComponent();
+        }
+    }
+}