Add keycodes support

[pdesaulniers]

This is an initial attempt at adding keycodes support to DPF.

These allow to deal with keyboard keys in a layout-independent manner. It's particularly useful for piano-style widgets, to map keyboard keys to piano notes in the correct order.

This code is based on https://github.com/depp/keycode, especially the <os>_tohid.c files and keycode.h in the src directory.

Tested with the piano widget on Linux, MacOS and Windows, with a couple of different layouts (Colemak, AZERTY, QWERTY).

The keycodes would be wrong when running the UI in Wine, but they would work fine on a real Windows 10 machine. I'm not sure if this is normal.

Perhaps it would be good to test this feature a little more before merging.

[falkTX]

Looks good, but a few things to do:

1. rename COPYING into COPYING.keycode
2. make the keycode arrays static
3. make KEYCODE_EVDEV_OFFSET static too, and check if opcode < KEYCODE_EVDEV_OFFSET before doing the -= operation (x11 code)

there are a little oddities regarding spaces vs tabs, but I can fix them later myself

also, exposing the keycode API would be nice yes :)

[pdesaulniers]

OK, I've made the changes.