Refactor registration of OOjs UI resource modules

Refactor registration of OOjs UI resource modules

includes/resourceloader/ResourceLoaderOOUIModule.php
* New trait centralizing some logic for dealing with OOjs UI themes,
  previously duplicated in OutputPage, ResourcesOOUI.php and
  ResourceLoaderOOUIImageModule.
* Follow-up change I74362f0fc215b26f1f104ce7bdbbac1e106736ad uses this
  as a base to allow skins/extensions to define new OOjs UI themes.

resources/Resources.php
resources/ResourcesOOUI.php
includes/resourceloader/ResourceLoader.php
* OOjs UI resource module definitions are moved back to their rightly
  place in Resources.php. They are again (almost) normal and static.
* Theme-specific logic is now handled by the module code, definitions
  only specify 'themeScripts'/'themeStyles'/'themeImages'.
* ResourcesOOUI.php is deleted and no longer loaded by ResourceLoader.

includes/resourceloader/ResourceLoaderOOUIFileModule.php
includes/resourceloader/ResourceLoaderOOUIImageModule.php
* Glue code previously existing in ResourcesOOUI.php now lives here.
* Use the ResourceLoaderOOUIModule trait to avoid code duplication.

Change-Id: I39cc2a735d9625c87bf4ede6f5fb0ec441d47dcc