Implement undo and redo for the ie and gtk webkit backends. Extend the sample to show their use.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/SOC2011_WEBVIEW@68135 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/webview_webkit.h

@@ -128,6 +128,12 @@ public:
     virtual void Copy();
     virtual void Paste();
 
+    //Undo / redo functionality
+    virtual bool CanUndo();
+    virtual bool CanRedo();
+    virtual void Undo();
+    virtual void Redo();
+
     /** FIXME: hack to work around signals being received too early */
     bool m_ready;