Return a list of dependencies for an object of a
particular class. For widgets, this method is defined by defwidget
automatically. If you don't use defwidget or if you want other
renderable types to have per-class-dependencies, you need to take care
to define this method for your class.

The default implementation for widgets uses the following protocol to
determine if a widget has dependencies. It looks under
*public-files-path*/scripts/[attributized-widget-class-name].js and
*public-files-path*/stylesheets/[attributized-widget-class-name].css. If
it finds the aforementioned files, it returns them as
dependencies. This way the developer can simply place relevant files
in the appropriate location and not worry about specializing this
function most of the time.