Decorator for marking plugin methods as callable from scene objects or representations associated with that plugin.
The method should accept an scene object/representation as its first argument after self. When the method is called
with the scene object/representation as the receiver, a proxy callable is returned which places the receiver as the
first argument followed by whatever other arguments are given. This allows calls of the form obj.plugin.foo(obj,...)
to be replaced with obj.foo(...). Decorated methods have a new member '__isdelegatedmethod__' containing True. An
overridden method will still be treated as a delegated method even if it does not have this decorator applied.