Improve scroll wheel speed by basing it on label height, not icon height

Summary:
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 based on the text label rather than the
icon size just like D25683, ensuring that the scroll speed does not vary and become
super fast when using large icons in particular.

It also reverts 90beb4a5, which is no longer needed.

BUG: 386379
FIXED-IN: 19.12.1

Test Plan:
Use a mouse with a scroll wheel and scroll in Dolphin item views with list view,
details view, icon view, etc, using different item sizes. Speed should be
consistent in all views now, and also feel consistent with other KDE apps.

Also try with multiple scale factors to make sure the behavior does not change.

No change with high-resolution two-finger touchpad scrolling.

Reviewers: #dolphin, elvisangelaccio

Reviewed By: #dolphin, elvisangelaccio

Subscribers: ahiemstra, lots0logs, anthonyfieroni, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D19190