Allow additional extraAllowedContent rules in CKEditor configuration

The CKEditor configuration property 'extraAllowedContent' makes it possible to configure extra rules for the client-side HTML cleaner builtin to CKEditor. However, the hardcoded default configuration of rich text fields also defines this property, as a string. So if projects want to add additional rules to this property, they should use the 'ckeditor.config.appended.json' configuration of a document field. However, that does not work for this particular property because string properties are appended to each other with a comma as separator. Yet the 'extraAllowedContent' property uses a semi-colon to separate rules. 

This can be fixed by changing the default configuration to the object notation. With that in place, people can use the ckeditor.config.overlayed.json property to add additional entries to the extraAllowedContent object. It'll also be possible to add extra allowed attributes via the ckeditor.config.appended.json property.