Implement support for scroll button locks

Merged Peter Hutterer requested to merge whot/xf86-input-libinput:wip/scroll-button-lock into master

Add a boolean option/property to enable/disable the scroll button lock. Where enabled, the button can be clicked and released as opposed to having to be held down.

Requires the libinput branch from libinput/libinput!191 (closed)

cc @pcewing

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