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.

Start by reading traitlets/traitlets.py and inventorying its public classes and functions against the documented traitlets API, including the accidentally exposed DefaultHandler. Done means the module defines an explicit __all__ containing the intended public API while leaving undocumented utilities unlisted.

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