Add RoutedCommand to PopupBox - Issue:#1191 (#1195)

* Add RoutedCommand to PopupBox

I think I have followed the requests, but I am happy to learn from my mistakes.

* Fix CommandBindings

* Fixes

Adds the null check in _popup and updates the comment on the RoutedCommand.

Author: alonfnt

MainDemo.Wpf/Buttons.xaml

@@ -155,8 +155,8 @@
                                 </ComboBox>
 
                                 <StackPanel Grid.Row="5" Grid.ColumnSpan="2" Orientation="Horizontal" HorizontalAlignment="Right">
-                                    <Button Content="_Save" />
-                                    <Button Content="_Cancel">
+                                    <Button Content="_Save" Command="{x:Static materialDesign:PopupBox.ClosePopupCommand}" />
+                                    <Button Content="_Cancel" Command="{x:Static materialDesign:PopupBox.ClosePopupCommand}">
                                         <Button.Style>
                                             <Style TargetType="Button" BasedOn="{StaticResource MaterialDesignPopupBoxButton}">
                                                 <Setter Property="Foreground" Value="Red" />

MaterialDesignThemes.Wpf/PopupBox.cs

@@ -103,6 +103,12 @@ public class PopupBox : ContentControl
         public const string PopupContentControlPartName = "PART_PopupContentControl";
         public const string PopupIsOpenStateName = "IsOpen";
         public const string PopupIsClosedStateName = "IsClosed";
+
+        /// <summary>
+        /// Routed command to be used inside of a popup content to close it.
+        /// </summary>
+        public static RoutedCommand ClosePopupCommand = new RoutedCommand();
+
         private PopupEx _popup;
         private ContentControl _popupContentControl;
         private ToggleButton _toggleButton;
@@ -394,7 +400,9 @@ public override void OnApplyTemplate()
             _popup = GetTemplateChild(PopupPartName) as PopupEx;
             _popupContentControl = GetTemplateChild(PopupContentControlPartName) as ContentControl;
             _toggleButton = GetTemplateChild(TogglePartName) as ToggleButton;
-            
+
+            _popup?.CommandBindings.Add(new CommandBinding(ClosePopupCommand, ClosePopupHandler));
+
             if (_toggleButton != null)
                 _toggleButton.PreviewMouseLeftButtonUp += ToggleButtonOnPreviewMouseLeftButtonUp;
 
@@ -440,6 +448,11 @@ protected override void OnMouseEnter(MouseEventArgs e)
             base.OnMouseEnter(e);
         }
 
+        private void ClosePopupHandler(object sender, ExecutedRoutedEventArgs executedRoutedEventArgs)
+        {
+            IsPopupOpen = false;
+        }
+
         private void OnLayoutUpdated(object sender, EventArgs eventArgs)
         {
             if (_popupContentControl != null && _popup != null &&