Create decorator alternatives for magic names

Just like Traits, there are a few places where TraitsUI makes use of specially-named methods. At least two of these are:

- https://docs.enthought.com/traitsui/traitsui_user_manual/handler.html#extended_traitname_changed
- `[extended_traitname]_setattr` (See #1564)

A better, more modern strategy would be to provide a method decorator which marks methods as handlers for these use cases. For example, something like:
```
@handler_setattr('model.foo')
def set_model_foo(self, info, object, name, value):
...
```

This would need to be coupled with a registry of these methods on the handler.

Eventually, after some time, the magic names could be retired.