Other changes

• Aligned ElementDefinition usage to the changes in the engine. See ckeditor/ckeditor5#742. (96d24f4)
• Increased the specificity of CSS rules. Introduced the .ck class for editor UI components (see: ckeditor/ckeditor5#494). (915ec69)

BREAKING CHANGES

• In the custom format of the heading feature configuration the view.style, view.class and view.attribute properties are now called view.styles, view.classes and view.attributes.