Added IsOfStandardClass() as per wxMSW.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@45673 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/window.h

@@ -260,6 +260,9 @@ public:
     GtkWidget           *m_widget;          // mostly the widget seen by the rest of GTK
     GtkWidget           *m_wxwindow;        // mostly the client area as per wxWidgets
 
+    // return true if the window is of a standard (i.e. not wxWidgets') class
+    bool IsOfStandardClass() const { return m_wxwindow == NULL; }
+    
     // this widget will be queried for GTK's focus events
     GtkWidget           *m_focusWidget;