add FlexPanel demo

src/Shared/HandyControlDemo_Shared/UserControl/Controls/FlexPanelDemoCtl.xaml.cs

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