DolphinView: Conform to global scroll speed

DolphinView: Conform to global scroll speed

One scroll with the mouse wheel is supposed to scroll the view by
QApplication::wheelScrollLines, however previous to this commit Dolphin
scrolled the view by QApplication::wheelScrollLines^2 instead, which
was wrong and way too much.

This commit fixes this by defining one line height as the height of the
current default font. This value is multiplied by
QApplication::wheelScrollLines to determine the scroll amount per mouse
wheel scroll.

In details view mode however, where there really are rows to go by,
this commit makes sure to always scroll by full rows. The number of
rows to scroll is determined by rounding up from the scroll amount used
in the other view modes.

Co-authored-by: Felix Ernst