Required args in panels_page_admin_view

Arguments that occupy a %-placeholder are de-facto required arguments.

The enclosed patch takes that into account by:
- calculating $required_args.
- displaying the link to the "Real URL" only if ($required_args > given $args), otherwise a hint 'Arguments missing'.
- marking the first $required_args argument textfields as '#required'.

This doesn't only improve the usability of the admin view page, more importantly it gives the user the feedback he needs to understand some of the most central concepts.

Also, $form['fieldset']['url'] is defined as '#type' = 'item', to ensure it is wrapped in a div.

Check it out...