Redesigned the set up of the user function arguments in the GUI pages.
The wizard element_*() methods have been removed as these were pointless wrappers. The Uf_page
add_contents() method now builds the argument GUI elements and stores them in the self.uf_args
dictionary. The wizard methods Clear(), GetValue(), ResetChoices() and SetValue() have been shifted
into the Uf_page class as these are only for the auto-generated user functions.
