Fix slow scrolling in dock panels
Commit f688bcd1f1 fixed slow scrolling with xf86-input-libinput on DolphinView.
However the commit also exposed a bug in the Dolphin scrolling
algorithm, which was previously hidden. This resulted in slow
scrolling in dock panels (Places and Folders), with both
xf86-input-evdev and xf86-input-libinput drivers, as well as libinput on
KItemListContainer::updateScrollOffsetScrollBar() relied on the view's
itemSize() method to compute the scrollbar's singleStep, but this QSize
was invalid for the dock panels' views.
We use a new itemSizeHint() method instead, which is always valid and
also adapts to the current icon size set in the view.