Fixes to Requirements and themes

* Make plugin::add_javascript and plugin::add_stylesheet work again
  Reverting some changes, and add function to get just filenames back
* Add Themes->plugin_requirements() and call this after other requirements
  are loaded.
* Check api_url isn't empty before adding to requirements
* Fix Requirements handling of IE CSS files
* Only add admin combine files groups if we've actually enabled admin
  theming