Move the code in  WKHtmlWindow.__init__ to an event handler for the EVT_WINDOW_CREATED event.

Fix for "where the window variable is returned as NoneType inside the wxGTKWebKit code, and the application will fail when trying to run get_user_data() on a NoneType object." from http://goo.gl/cVZ72. The idea for the fix came from http://goo.gl/9Ked3.