add close event for wx timer app in loop_wx

[newville]

This fixes #1477 by adding a "close event" to the wxPython timer app.

The wx close event calls wake one final time, and cleanly stops the Timer, and destroys the wx Frame.

[ianthomas23]

Thanks @newville, it looks good to me. I'll defer to your wx knowledge that this is fine from a wx point of view.

Linting failure is unrelated so I will fix that separately.

[newville]

@ianthomas23 Thanks for the very quick response!