Firefox no longer sends keyup events properly for the Alt keys. Try
to sniff out the state of the Alt key by monitoring other events that
include its state.
Use normal properties with JavaScript setters and getters instead of
our homegrown stuff.
This also changes the properties to follow normal naming conventions.
Setting a style to null does restore it in FF, Chrome, Safari and Edge.
But it does not work in Internet Explorer. The proper way to restore to
default values is to set it to the empty string. This works in all
browsers. Fixes issue #808.
This commit restructures many of the ES6 modules, splitting them
up to actual export multiple functions instead of a single object.
It also splits up Util into multiple sub-modules, to make it easier
to maintain.
Finally, localisation is renamed to localization.
