Split workflow.module

Modules for Drupal 6 normally put the menu callbacks in <module_name>.admin.inc (administration pages), and <module_name>.pages.inc (other pages).
This reduce the code the PHP interpreter must parse when the module is loaded (which happens all time Drupal bootstraps), and it makes Drupal faster (the <module_name>.module is always included, whenever the module does something, or it does nothing at all).