Don't warn about missing numpy in traitsui.api

In an environment that doesn't have NumPy installed, importing `traitsui.api` gives a warning:

```
/Users/mdickinson/.venvs/traits-py38/lib/python3.8/site-packages/traitsui/editors/api.py:15: ImportWarning: ArrayEditor is not available due to missing numpy
```

I don't think this should be a warning, or at least not an import-time warning: it's legitimate to have need of a Python environment that has TraitsUI but not NumPy.

One option would be to define `ArrayEditor` as a stub in the absence of NumPy, and have that stub warn or raise on first use.