add style and demo

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

@@ -1,4 +1,6 @@
-namespace HandyControlDemo.UserControl
+using System.Windows;
+
+namespace HandyControlDemo.UserControl
 {
     public partial class SplitButtonDemoCtl
     {
@@ -6,5 +8,10 @@
         {
             InitializeComponent();
         }
+
+        private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
+        {
+            MessageBox.Show("123");
+        }
     }
 }