Reduce button scroll timeout to 38ms

When using button scrolling, a hardcoded delay of 200 milliseconds between button down and scroll events being emitted makes fast scrolling gestures feel clunky and sometimes fail entirely. This feature comes from xf86-input-mouse, was copied into xf86-input-evdev and reimplemented in libinput.

This was, as far as can be determined, to allow right clicks without triggering scrolling. libinput now also has distance triggers (2bbf4a01) and sends button events if no movement has happened for long clicks, regardless of the delay.

The 200ms delay is thus not really necessary anymore, let's drop it to 38ms which is just above the 3-event threshold for 8/10/12ms intervals which is most devices.

Fixes #237

This branch supersedes !179 (closed)