Implement a touchpad scroll distance property

To be used for touchpads and continuous (i.e. button-based scrolling).

libinput provides us with pixel data for finger-based and button-based
scrolling but the X server does support this - XI2.1 smooth scrolling is
merely centered around a logical scroll click (defined as "increment"), with
smooth scrolling being a fraction of that increment. For example, in the old
synaptics driver that value was in device-specific units and thus different
for every device.

The increment is a constant value set in the ScrollClass and cannot be changed
at device runtime. So we simply initialize with a random default (15, because
that works well for wheels) and then scale our pixel delta in to that range.

With the default value, a 15 pixel movement would result in a logical scroll
click, if the distance is set to 30 the users has to move 30 pixels to trigger
that scroll click.

From the client's perspective nothing changes, the increment is still the
same.

Range checks are quite restrictive, this option is supposed to improve
usability, not as a workaround around other bugs.

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