Summary:
A surface has an API to know which screen(s) it's on, this is useful
especially for knowing the scale it should render at.

In practice Qt currently doesn't do anything with this information; but
that's set to change.

Test_helpers is changed as we need to create output objects in order for
wl_surface to map them.

Closes task T4467

Test Plan:
Attached unit test.
As mentioned above, it doesn't have any real world impact currently, so not a lot
to test.

Reviewers: #plasma, graesslin

Reviewed By: #plasma, graesslin

Subscribers: johanhelsing, graesslin, plasma-devel, kwin, #kwin

Tags: #kwin

Maniphest Tasks: T4467

Differential Revision: https://phabricator.kde.org/D7359