Reconsider our use of `__getattr__()` hackery for method inheritance

[jmurty]

We use the __getattr__() special method in a couple of places in GLAMkit to do something resembling method overriding, except calling some default method implementation if no other class overrides the method in the correct way (i.e. by defining the method on a subclass).

See examples in ListableMixin and EventBase.

This whole approach makes me nervous since it is hacking around Python's standard MRO class-inheritance method overriding behaviour for reasons that are not clear to me.

Sooner or later we are likely to be bitten hard by this unusual behaviour, beyond just the work-arounds I have already need for f0a5499 for example. Can we scrap these __getattr__() methods altogether?