Move the x11 plugin sources into a x11/windowed directory

Preparation step for having two x11 backends: the windowed for Wayland
and a standalone for kwin_x11.

Add a very basic x11/standalone platform plugin

The plugin does not much. It's the most basic plugin we can have to
be loaded from kwin_x11.

Unlike the wayland platform plugins it gets installed to:
org.kde.kwin.platforms

Load and init the KWinX11Platform plugin in main_x11

kwin_x11 now also loads a platform plugin and initializes it.
It doesn't do much except loading it and init it. Though it's also
set up to continue startup only when screens are queried.

Move XRandrScreens to the x11/standalone plugin

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.