with the switch to GTK2. This turns out to be because, where GTK1
represented the scroll wheel as mouse buttons 4 and 5 and generated
GdkEventButton when it was moved, GTK2 has moved wheel actions out
into a new event type GdkEventScroll which we were not handling. Now
we do, so scroll wheel support should be back in place.
[originally from svn r8063]