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

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>