Make the item heights rounded to avoid font rendering issues

Summary:
Fixes small font rendering issues produced by items which are offset by
half a pixel because of multiplying `gridUnit` with `2.5`. This is especially
noticeable with bitmap fonts where parts of the letters are missing.

Reviewers: mart, davidedmundson, broulik, ngraham

Reviewed By: davidedmundson, ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11502