Fix qt help button dialog

The help button did not work so I changed the qt4/ui_base.py onhelp 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 i was previously written to use 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).