Consider the primary screen as default screen

Summary:
We used to append them, but that didn't work well and was crashing plasmashell
on fresh start.
A default desktop would be created alongside with the one provided by the
layout instead of replacing it.
If a layout wants to provide an additional screen for a desktop in the secondary
screen, it should specify the screen.

Test Plan:
Now the plasmashell tests pass. In fact I noticed it was broken due to an e-mail
Jonathan sent me that the test on neon was timing out. The test in neon will
freeze when the test crashes. Probably something to look into.
It can be reproduced by running:

xvfb-run -a --server-args="-screen 0 1024x768x24" dbus-launch --exit-with-session <exec>

Where exec is the process we need to run.

Now the test passes.
It's a crash that I had reproduced locally in the past. I can't now.

Reviewers: Plasma

Subscribers: plasma-devel, Neon

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2117