Merge branch 'prototype-1.7.3' into 'master'

lib: Upgrade Prototype from 1.7 to 1.7.3

Are you sure you want these as separate merge requests?  The rest of the upgrades are much less invasive than Dojo, with essentially no changes outside of `lib`.  I would of course leave them as separate commits in any case, but I had assumed they would be more convenient to test in one batch.

See merge request !39