Adding the 'setUnits' function

Summary: Added the function per se, an enum for the metric system, and the function declaration.

Reviewers: Atelier: AtCore, patrickelectric, rizzitello, laysrodrigues

Reviewed By: Atelier: AtCore, patrickelectric, rizzitello, laysrodrigues

Subscribers: rizzitello

Tags: Atelier: AtCore

Differential Revision: https://phabricator.kde.org/D6040