[Milou] Take label height into account for delegate height

Font size and icon size are independent from each other (in contrast to
spacing/gridUnit which depend on font) so it can happen that the font is
disproportionately large compared to the icon resulting in squashed delegates.

BUG: 352696
FIXED-IN: 5.6.5

Differential Revision: https://phabricator.kde.org/D1660