Box holes have drop feedback

Hole is highlighted not the whole box as previously. Also improved
highlighting feedback.
Fixed a bug removing widgets

If the widget's backside didn't implement removed_from_container
Switched to addEventListener from on

JQuery's on interferes with adding multiple listeners and also adds code
complexity with event.originalEvent.
Next robot starts training on drop

When a fresh robot is dropped on a robot's next robot drop area it
starts training.