use observe in traitlets.config

[minrk]

instead of deprecated APIs

[minrk]

This has already broken the IPython kernel. I need to make sure to update this such that _config_changed accepts both the @observe and pre-@observe signatures.

[SylvainCorlay]

It is clear that if someone registers @observe('_foo_changed') and you override the method in derived classes, the overridden method will not be called. Do you want to give priority to the old API in the implementation so that whenever we have a _foo_changed method, it is called with priority over the registered one?

[minrk]

For this to be valid as 4.1, we need Configurable subclasses that have something like:

def _config_changed(self, name, old, new):
    ...
    super()._config_changed(name, old, new)

to work exactly as before. This should also work in such a way that the base Configurable class doesn't produce any DeprecationWarnings for having _config_changed defined (it's fine for the downstream case to produce the warning if the method isn't decorated).