As elsewhere, make sure objects that need to have a ownership up to the
context, and use this ownership chain to find relevant components, such
as the backend or the Wayland compositor object instance.
wayland/data-device: Hook up data devices to seats
They are tied to a seat - make that connection in struct fields too, so
that related objects can get to the context via it.
wayland: Don't get Wayland compositor via singleton getter
This means via the ownership chain or equivalent.
xwayland: Hook up manager to Wayland compositor
Same applies to the drag-n-drop struct.
xwayland: Make X11 event handling compositor instance aware
This avoids finding it via singletons in the callee.
xwayland: Don't get Wayland compositor from singleton
xwayland: Pass manager when handling dnd event
window/xwayland: Don't get Wayland compositor from singleton
xwayland/grab-keyboard: Don't get backend from singleton
xwayland: Don't get backend from singleton
wayland: Always get the backend from the context
This means traveling up the ownership chain or equivalent when
necessary.
wayland: Hook up data devices, offers and sources to the compositor
This allows tying them to a context without going through any
singletons.
wayland: Don't get display from singleton
xwayland: Don't get display from singleton
tablet: Don't get display from singleton
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2718>
