changed _on_help method to pass None for control (as default)

The help button did not work so I changed the qt4/ui_base.py _on_help method which was trying to pass an undefined event attribute.  Since the follow-on methods and take None,  I added a event=None default argument (in case there was some reason to take an event), and then used it to generate a control or None value to pass to the rest of the help chain which want (ui.info, control) arguments.  Now the Help button creates a help dialog (for qt at least).