Wx Event Handler clean-up issues

There are issues with the wxPython 4 backend where event handlers are not being cleaned up correctly. It appears that all of the event handlers explicitly added by TraitsUI are being dealt with appropriately, but there appear to be some others which are internal to wxPython with aren't being cleaned up:

- scrollbars and some base Window objects seem to have an `EvtHandler` associated with them which needs to be cleaned up.
- the Html widget appears to have an `EvtHandler` associated with it that causes an error if it _is_ cleaned up.

This is easy to hack a fix for, but I suspect a deeper problem with finalization.