Support for extra mouse buttons
As SPICE is being used more and more for general desktop interactivity through tools like Looking Glass, the demand is growing for support for the additional buttons on the mouse. Namely the 'Back' and 'Forward' buttons. QEMU refers to these buttons as
EXTRA, as such I suggest the naming convention be kept the same for simplicity.
Since the protocol is using a button mask, adding defines and support for these additional buttons would be trivial and would not break backwards compatibility support.