Dolphin currently scrolls by the height of three items at a time per "step" when
using a scroll wheel. Because item height is highly variable, this leads to scroll
speed being inconsistent between views, and generally far too fast when using
icon view with icons larger than 22px size.
This patch makes the size of the scroll step fixed, using the height of
2.5 text labels + their padding, which is what KDirOperator
does. This feels much better when using a scroll wheel mouse, and better
matches the scroll speed in other KDE apps.