New "ABS_WHEEL2" axis?

I'm working on adding support for the recently-announced Cintiq 24HD,
and am hashing out most of the details on the linuxwacom mailing list.
In the discussions there, it was suggested that we add a new e.g.
"ABS_WHEEL2" axis to the input.h header to represent the second touch
ring available on the 24HD. I think it's a good idea, but wanted to
get some opinions from over here before making a patch. The other
option we have available is to use an axis not already in use by the
wacom driver (such as ABS_RUDDER), but none have a matching semantic
meaning.

Jason

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