Load the skin CSS in parallel

[fredck]

We're currently loading the skin CSS files only after the plugins files get loaded. This is not really necessary, and these files could be loaded in parallel, while the rest of the editor scripts get loaded.

[fredck]

The solution for it could be quite simple. It's enough to (unblocked) load the skin files as the first thing right after loading the configuration file.

[fredck]

For reference, this idea comes from here:
http://ckeditor.com/blog/CKEditor_Loading_performance_details#comment-109

[garry.yao]

Result Comparison

[fredck]

The callback function at line 256 should not be removed. It'll not block for CSS files. It blocks only if the skin.js file needs to be downloaded.

Inside the above callback, we must call loadLang now, removing the call for it at line 130.

[garry.yao]

Depends on #4463.

[garry.yao]

Fixed by #4463.