explicit list exported traits with `__all__`

We should define `__all__` in traitlets/traitlets.py, so that it's explicit what's exported as public API.

There are a few utility functions and classes such as DefaultHandler that are accidentally exposed (undocumented, so nobody has any reasonable expectation of using them). We should take the opportunity of a 5.0 release to explicitely list the actual traitlets API.