Update CKEditor library to 4.1 RC and remove CKEditor default style configuration

Release announcement: http://ckeditor.com/blog/CKEditor-4.1-RC-Released

Added to core at #1878344: Add CKEditor JS library to core and updated at #1905424: Update CKEditor library, we should now update it again. The 4.1 RC release marks the stability of the Advanced Content Filter functionality that CKEditor implemented upon our request. That's also the flagship feature of this release.

So, why update? For several reasons:

1. http://dev.ckeditor.com/ticket/9994 is part of 4.1 RC, which allows us to implement #1894644: Unidirectional editor configuration -> filter settings syncing, which is a major task, and (probably) the last CKEditor integration aspect for which there is not yet a patch.
2. Currently, CKEditor always loads its styles.js file. However, we don't need/use it. This has been fixed in 4.1 RC: http://dev.ckeditor.com/ticket/9992#comment:4. It allows us to remove that file from our repository, and removes 1 unnecessary HTTP request. WPO FTW.
3. Source mode editing when using in-place WYSIWYG editing, thanks to 4.1 RC's new "source dialog" plugin. (That opens up a dialog to edit HTML source code.) See #1886566-34: Make WYSIWYG editors available for in-place editing.
4. Users can test Advanced Content Filter, potentially leading to bug reports for CKEditor. We already have it, but then we'll have a more mature version of it.

Relevant detail: to be able to not load styles.js when not using the "Styles" button/dropdown, we must set stylesSet = FALSE, I had to make the StylesCombo plugin implement CKEditorPluginContextualInterface: when the button is not enabled, it will set stylesSet = FALSE. See interdiff_php.txt for that.

With this new build, I optimized & clarified the build-config.js file again (which determines which plugins are included in the build). See interdiff_build-config.js.txt for that.

This means the CKEditor Widgets are temporarily unavailable (they're not part of the 4.1rc tag or the major branch, and the d8 branch doesn't contain the ACF improvements), but that's okay, we were not yet using that yet anyway, and they will be available again soon, when we update the build again. This update addresses all pressing concerns regarding CKEditor integration work.