Mouse wheel event handling has now been standardised and has broad
support. Use this event rather than the older, non-standard ones.
At the same time fix up support for horisontal mouse wheel events.
This commit moves all the input-related files from `core/`
to `core/input/`, and renames a couple as relevant
(input.js --> input/devices.js, keyboard.js --> input/util.js).
Samuel Mannehed
Pierre Ossman
zarmhast
Solly Ross