Wheel event improvements

[almarklein]

• [x] Reduce wheel event throttle timeout. Closes #48.
• [x] Document wheel event better.
• [x] Prevent unintentional consumption of the wheel event. Closes #47.

[djhoese]

If you aren't customizing the API docs too much you may want to look into sphinx apidoc: https://www.sphinx-doc.org/en/master/man/sphinx-apidoc.html or https://pypi.org/project/sphinxcontrib-apidoc/

Might save you time in the future.

[almarklein]

To prevent the widget from unintentionally consuming wheel events, there are (at least) two options:

1. Keep track of the object having focus (having been clicked on).
2. Check if the wheel event originated from over the widget, using a timeout mechanism (a technique I've used in Flexx).

I initially went with (2), see the commit, but then I remembered that we already keep track of focus for key events, so (1) is easy to implement, and that approach also feels more consistent to the user.

[almarklein]

Ready!