[kwin_wayland] Initial addition of the WaylandServer module

So far this new module contains:
* Display
* OutputInterface

Display manages the server socket and server event loop. In general it's
the entry point to any part of the server.

OutputInterface is the abstraction for the wl_output interface on server
side. An OutputInterface is created through the Display.

The auto tests for ConnectionThread and Output are adjusted to use the
internal server instead of starting Weston. Especially the Output test
could be extended to test much more as we have absolute control over
the server now.