Show unconnected displays at the right side of the main screen

Summary:
Until now they were shown half behind the center (main) screen.
This makes it nicer, although with many screens it might flow too much to the right.
For this reason, reduce the visual size of the screens.
More work in this sense will follow.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D17682