By moving XRandrScreens the creation of screens gets simplified a lot
as there is no need to have windowing system specific init code. It all
just goes through the platform.
This also marks the point where the first X11 specific code is removed
from kwin_wayland.
Reviewers: #plasma, sebas
Subscribers: plasma-devel
Projects: #plasma
Differential Revision: https://phabricator.kde.org/D1355
