Physically based kinetic scrolling

[Lieven van der Heide <lievenvanderheide gmail com>]

I modified the scrolled windows kinetic scrolling to match the actual physics laws for friction and springs. IMHO, this gives a much more pleasing result. 

A branch with these changes is at

https://github.com/lievenvanderheide/gtk-kineticscrolling

I hope that someone can have a look at it, and maybe merge it into the main gtk branch.

Regards,

Lieven van der Heide.