[Folder View] use Math.floor() instead of Math.round()

Summary:
This is more technically correct and doesn't leave any empty space on the right side
of the screen.

Test Plan: {F6763177}

Reviewers: hein, #plasma

Reviewed By: hein, #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D20420