Move the menu callbacks outside the module file

Issue Summary

Drupal 6 modules put the menu callbacks outside the module file, generally in files called <module_name>.admin.inc (administration pages), or <module_name>.pages.inc (normal pages). This permits to reduce the code loaded at runtime when Drupal is loaded, and permits the web site to run faster.

This is made possible by a new attribute, which permits to tell to Drupal from which file the callback function must be loaded (it's 'file').