Create editor#addContentsCss() method

[Marek Lewandowski]

We should create more convenient way to add custom css files. Now we accept as editor.config.contentsCss string or array of strings which is ok, but if we want to attach to it extra css styles in plugins, we have to do some extra checking. Such code is redundant.

Method should be editor member, so we will end up with editor.addContentsCss(). Method itself should be injected from wysiwygarea plugin, since contentsCss works only for classic editor.

Method should allow to accept as its parameter both string and array of stylesheet paths.