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.