compositor: Custom WaylandSurfaceItem for shell windows

Shell windows don't need a window container we just implement a custom
WaylandSurfaceItem with a surface renderer.

In the future we will determine position and z-index by reading the
window properties from the surface.

Closes #106
compositor: Use custom surface item from Green Island

Green Island now provides a custom surface item which does all what we
implemented here and will be the standard implementation for all custom
compositors.

We don't need a WindowContainer and a WindowChrome anymore, this makes
things a lot easier.

Issue: #105