A BSD-licensed clone of RISCOS Ltd's WindowScroll module, based on the
documentation at
http://www.riscos.com/support/developers/riscos6/input/windowscroll.html

Currently it's a bit basic - future improvements could include:

* Have the "Speed" setting affect line scroll mode (read back the new
window position and estimate how much further we can scroll it before
overshooting - difficult because the scroll/redraw events are buffered
by the Wimp)
* Momentum/acceleration to ease scrolling long distances

Version 0.01. Tagged as 'WindowScroll-0_01'