Make adding resources and css classes easier

[Cito]

TW 1 supported a css_classes parameter to add css classes in addition to css_class. TW 2 only has the css_class parameter which makes it difficult to add CSS classes to an existing parent class. You need to write something like css_class = ParentWidget.css_class + " another_class"in order to not overwrite an existing parent class.

A similar problem exists concerning the resources parameter. Usually you want to just add resources to a parent widget, not replace the parent resources completely.

Maybe we could add parameters like add_css_class and add_resources for this purpose? When using these parameters, the classes/resources would be just added to existing base classes/resources.