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/webview.h

@@ -333,6 +333,12 @@ public:
     virtual void Cut() = 0;
     virtual void Copy() = 0;
     virtual void Paste() = 0;
+
+    //Undo / redo functionality
+    virtual bool CanUndo() = 0;
+    virtual bool CanRedo() = 0;
+    virtual void Undo() = 0;
+    virtual void Redo() = 0;
 };
 
 class WXDLLIMPEXP_WEB wxWebNavigationEvent : public wxCommandEvent