Allow chaining the after calling addClass

[Alfonso Martínez de Lizarrondo]

The ​http://docs.ckeditor.com/#!/api/CKEDITOR.dom.element-method-addClass should include a

return this;

for symmetry with removeClass and to follow the same pattern that other frameworks like jQuery that allows chaining calls to the element.

[Jakub Ś]

There is nothing more to say :). I fully agree with this one.

[Piotrek Koszuliński]

Fixed on master with ​git:06bdff1b4.

However, I'm not a huge fan of chaining. It's a nice pattern and in some cases it helps a lot, but not much in ours. It reduces readability a bit, it reserves return values so one must be very careful when making new methods chainable, it requires additional code (return statements), etc. And there's no much profit from it. So, I don't think we'll start making most of our methods chainable.