Return a list of dependencies for a particular
object.

Whenever a widget is rendered by weblocks, this function is called to
determine which stylesheets, javascript files and javascript code the
widget depends on. These dependencies are then processed and included
in various places (header, body, AJAX page update) of the containing
page.

The default implementation for widgets finds some dependencies
automatically based on the widget's class name, see
'per-class-dependencies for details.

This method uses the append method combination to also return
dependencies for the superclasses of 'obj', because it's intuitive to
assume that a widget will use the stylesheets of its superclasses.