base scrolling on the smallest item

Summary:
CCBUG: 386379

after recent highdpi patches on scrolling that delegated it
completely to the scrollbar, based upon the scrollbar singleStep
setted to the tallest of the items in the view.
tough this makes scrolling way too fast, and on folders where just
few filenames are longer than most we can get a single scrolling
step almost double the number of lines configured in the
mouse kcm.
Using the shortest item instead of the tallest mitigates this problem
making it a bit more usable

Test Plan:
tested on different folders in different view modes both with
mouse and touchpad

Reviewers: Dolphin, broulik, ngraham

Reviewed By: Dolphin, ngraham

Subscribers: ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D10102