Move shared button identifiers from enum to an inline function, so formerly shared identifiers can be separately filtered and processed on a class-by-class basis.
Fixes#611.
The following collisions resolved:
hid.h -> usbhid.h
hid.cpp -> usbhid.cpp
HID -> USBHID
HID_BOOT_PROTOCOL -> USB_HID_BOOT_PROTOCOL
HID_PROTOCOL_NONE -> USB_HID_PROTOCOL_NONE
HID_PROTOCOL_KEYBOARD -> USB_HID_PROTOCOL_KEYBOARD
HID_PROTOCOL_MOUSE -> USB_HID_PROTOCOL_MOUSE
As a result, it's possible to use the library together with Arduino's bundled HID / Mouse / Keyboard libraries (Leonardo, Micro, or Due).
https://www.arduino.cc/en/Reference/MouseKeyboard
