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 build KWin's
Wayland binary optionally without X dependencies. Another long term goal, that
becomes possible through this separation is to recover from Xwayland crashes.