Move MetaWindow pointer to relevant roles

Quoting the last commit message

    wayland: Move MetaWindow ownership to window owning roles
    
    There are two surface roles owning a MetaWindow: MetaWaylandShellSurface
    (basis of MetaWaylandXdgToplevel, MetaWaylandXdgPopup,
    MetaWaylandWlShellSurface, etc), and MetaXwaylandSurface.
    
    With these two role types, the MetaWindow has two different types of
    life times. With MetaWaylandShellSurface, the window is owned and
    managed by the role itself, while with MetaXwaylandSurface, the
    MetaWindow is tied to the X11 window, while the Wayland surface and its
    role plays more the role of the backing rendering surface.
    
    Before, for historical reasons, MetaWindow was part of
    MetaWaylandSurface, even though just some roles used it, and before
    'wayland: Untie MetaWindowXwayland lifetime from the wl_surface' had
    equivalent life times as well. But since that commit, the management
    changed. To not have the same fied in MetaWaylandSurface being managed
    in such drastically different ways, rearrange it so that the roles that
    has a MetaWindow themself manages it in the way it is meant to; meaning
    MetaWaylandShellSurface practically owns it, while with Xwayland, the
    existance of a MetaWindow is tracked via X11.