The Xwayland code path is moved from ApplicationWayland to a dedicated class
Xwayland in a new top-level directory xwl.
This is a direct preparation step for generic support of Xwayland Selections.
On a longer timescale this should also allow us to further separate Wayland
native functionality from Xwayland to allow us at one point to optionally run
the Wayland session without Xwayland. Another long term goal becoming possible
through this separation is to recover from Xwayland crashes.