wayland: Untie MetaWindowXwayland lifetime from the wl_surface
For the most part, a MetaWindow is expected to live roughly as long as the associated wl_surface, give or take asynchronous API discrepancies.
The exception to this rule is handling of reparenting when decorating or undecorating a window, when a MetaWindow on X11 is made to survive the unmap/map cycle. The fact that this didn't hold on Wayland caused various issues, such as a feedback loop where the X11 window kept being remapped. By making the MetaWindow lifetime for Xwayland windows being the same as they are on plain X11, we remove the different semantics here, which seem to lower the risk of hitting the race condition causing the feedback loop mentioned above.
What this commit do is separate MetaWindow lifetime handling between native Wayland windows and Xwayland windows. Wayland windows are handled just as they were, i.e. unmanaged together as part of the wl_surface destruction; while during the Xwayland wl_surface destruction, the MetaWindow <-> MetaWaylandSurface association is simply broken.
WIP because it depends on !773 (merged).