A decorator to build a CallbackProperty.

This is used by wrapping a getter method, similar to the use of @property:

class Foo(object):
    def x(self):
         return self._x

    def x(self, value):
        self._x = value

In simple cases with no getter or setter logic, it’s easier to create a CallbackProperty directly:

class Foo(object);
    x = CallbackProperty(initial_value)