Make button numbers consistent across platforms

There's no universal consensus on how buttons are numbered.  Left, right,
middle as 0, 1, 2 seems to be the most common convention on modern vaguely
similar libraries, so I've gone with that.

The switch to zero-based indices will obviously break all current client code.
Particularly since now is the time to finish any breaking changes before a
stable release, I think that is better than only changing the middle and right
numbers, which would likely go unnoticed.

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