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Keycodes are defined in `common/keycode.h`.
Range of 00-A4 and E0-E7 are identical with HID Usage:
-<http://www.usb.org/developers/devclass_docs/Hut1_11.pdf>
+<http://www.usb.org/developers/hidpage/Hut1_12v2.pdf>
Virtual keycodes are defined out of above range to support special actions.