give richtext editor widgets access to the popup, so they can store more data in the URL and get it back when the callback is fired

This is used by the ckeditor, where you not only need the callback name, but
also the editor function num, which is a separate query parameter on the popup.
unified the handling of additional URL parameters in mediapool/linkmap popups

This allows richtext editors with special widgets to configure a list of
additional parameters that Sally should keep track of. Plus it finally fixes
a lot of places where previously the 'args' parameter was simply lost.
fixed param confusion