[kwin_wayland] Move Wayland::Output into dedicated source files

At the same time adding an autotest for the Output, moving the listener
into the Output class and providing enums for Subpixel and Transform.

KWin now requires wl_ouput interface version 2 as that allows us to emit
the changed signal in a better way.

The unit test is not yet capable of testing everything, we need a mock
Wayland server which is more flexible.