Add simple PopupOpened/Closed event example to demo project

Found in "Buttons & Toggles", "PopupBox, Style MaterialDesignMultiFloatingActionPopupBox"

Author: Eric Rodewald

MainDemo.Wpf/Buttons.xaml

@@ -80,7 +80,9 @@
                 <materialDesign:PopupBox Style="{StaticResource MaterialDesignMultiFloatingActionPopupBox}"
                                          PlacementMode="BottomAndAlignCentres"
                                          ToolTipService.Placement="Right"
-                                         ToolTip="PopupBox, Style MaterialDesignMultiFloatingActionPopupBox">
+                                         ToolTip="PopupBox, Style MaterialDesignMultiFloatingActionPopupBox"
+                                         PopupOpened="PopupBox_OnPopupOpened"
+                                         PopupClosed="PopupBox_OnPopupClosed">
                     <StackPanel>
                         <Button ToolTip="One">1</Button>
                         <Button ToolTip="Two">2</Button>

MainDemo.Wpf/Buttons.xaml.cs

@@ -39,5 +39,15 @@ private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
 	    {
             Console.WriteLine("Just checking we haven't suppressed the button.");
 		}
+
+        private void PopupBox_OnPopupOpened(object sender, RoutedEventArgs e)
+        {
+            Console.WriteLine("Just making sure the popup has opened.");
+        }
+
+        private void PopupBox_OnPopupClosed(object sender, RoutedEventArgs e)
+        {
+            Console.WriteLine("Just making sure the popup has closed.");
+        }
     }
 }