scale property for KScreen::Output

Summary:
This adds a scale property to KScreen::Output. By default, it's 1.0,
which means no scaling. The value is a qreal, and the conversion to int
for the backends that only understand int is made in the underlying
backend code. RFC. :)

Test Plan: autotests included

Reviewers: graesslin, davidedmundson

Reviewed By: davidedmundson

Subscribers: plasma-devel, davidedmundson

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D3312