Fix #876: editor: Reference correct column during change handling

editor sets up a single delegated event handler for native input elements,
but it was always referencing the first column that it happened to set up
the handler for.  This happened to be mostly harmless in most cases, but
caused issues particular to radio button handling.
CHANGES.md: Note editor input change event handling fix
editor functional tests: minor consistency update

(Doesn't affect outcome of tests)