OOUI theme support

OOUI theme support

* Split off OOjs UI's module definitions to a separate file from
  Resources.php.
* Extend ResourceLoaderImageModule to support per-skin images and
  variants.
* Allow skins to specify their preferred OOjs UI theme using new
  skin registry attribute 'SkinOOUIThemes'. The default remains the
  'mediawiki' theme, 'apex' can also be chosen now.
* Implement custom ResourceLoaderOOUIImageModule which implements some
  special sauce required to shoehorn OOjs UI image set definitions into
  ResourceLoaderImageModule with skin-specific styles support.

Bug: T100895
Change-Id: I3fbacbce95126a4cc29221352f3ef846f41e7b1b