Better workaround to dragleave problem

By temporarily turning off events for all descendants.
Replace JQuery 'on' with 'addEventListener'

Because Chrome Developer tools Elements inspector has links to where the
events are added.
Drop feedback broke empty holes

So boxes no longer display drop feedback.
Canonicalise  element attribute values

Currently only angles are constrained to between 0 and 360. Will be good
for conditionals. Fixes #56.