[gtkmm-documentation] custom_widget example: Don't use Gtk::WidgetCustomSnapshot

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 019514aa9023f22bd8db15e1c6fefca0f8e045ab
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Thu Apr 19 09:03:54 2018 +0200

    custom_widget example: Don't use Gtk::WidgetCustomSnapshot
    
    WidgetCustomSnapshot has been removed. snapshot_vfunc() is now
    a member of Gtk::Widget.

 examples/book/custom/custom_widget/mywidget.cc |    2 +-
 examples/book/custom/custom_widget/mywidget.h  |    2 --
 2 files changed, 1 insertions(+), 3 deletions(-)
---
diff --git a/examples/book/custom/custom_widget/mywidget.cc b/examples/book/custom/custom_widget/mywidget.cc
index 9bf404c..0510424 100644
--- a/examples/book/custom/custom_widget/mywidget.cc
+++ b/examples/book/custom/custom_widget/mywidget.cc
@@ -16,6 +16,7 @@
 
 #include "mywidget.h"
 #include <gdkmm/general.h>  // for cairo helper functions
+#include <gtkmm/snapshot.h>
 #include <iostream>
 //#include <gtk/gtkwidget.h> //For GTK_IS_WIDGET()
 #include <cstring>
@@ -24,7 +25,6 @@
 MyWidget::MyWidget() :
   //The GType name will actually be gtkmm__CustomObject_MyWidget
   Glib::ObjectBase("MyWidget"),
-  Gtk::WidgetCustomSnapshot(),
   MyExtraInit("my-widget"), // CSS node name, which must be used in the CSS file.
   Gtk::Widget(),
   m_padding()
diff --git a/examples/book/custom/custom_widget/mywidget.h b/examples/book/custom/custom_widget/mywidget.h
index 6af3f19..8601681 100644
--- a/examples/book/custom/custom_widget/mywidget.h
+++ b/examples/book/custom/custom_widget/mywidget.h
@@ -19,12 +19,10 @@
 
 #include <gtkmm/widget.h>
 #include <gtkmm/cssprovider.h>
-#include <gtkmm/widgetcustomsnapshot.h>
 #include "myextrainit.h"
 
 class MyWidget
 :
-public Gtk::WidgetCustomSnapshot,
 public MyExtraInit,
 public Gtk::Widget
 {