Better class management

• class([String]), class(String?), class(String...) always sets a new value for the class list
• class(add: [String]), class(add: String?) add one ore more new values to the class list
• class(remove: [String]), class(remove: String?) remove one ore more values from the class list
• class(toggle: String?) toggle (add or remove) a single class value
• Node contents is a public variable instead of a constant (let -> var)
• Better unit tests for class manipulation & node content changes