py-AddOn classes are like dynamic mixins, but with their own private attribute
and method namespaces. A concern implemented using add-ons can be added at
runtime to any object that either has a writable __dict__ attribute, or is
weak-referenceable.
AddOn classes are also like adapters, but rather than creating a new instance
each time you ask for one, an existing instance is returned if possible. In
this way, add-ons can keep track of ongoing state. For example, a Persistence
add-on might keep track of whether its subject has been saved to disk.
06bf89c774