[Folder View] Improve label crispness

Summary:
Without rounding the result, sub-pixel rounding errors accumulate over time, making the
label blurry.

Test Plan: Nice and crisp now: {F6762000, size=full}

Reviewers: fvogt, #plasma, hein, filipf

Reviewed By: filipf

Subscribers: abetts, filipf, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D20407