As with other parts, make objects have the ability to walk up the
ownership chain to the context, to get things like the Wayland
compositor or backend instances.
Contains these squashed commits:
display: Don't get backend from singleton
window: Don't get backend from singleton
keybindings: Don't get backend from singleton
workspace: Don't get backend from singleton
display: Don't get Wayland compositor from singleton
selection: Add display getter
context/main: Get backend directly from the context
clipboard-manager: Don't get display from singleton
stack-tracker: Don't use singleton MetaLater API
startup-notification: Hook up sequences and activations to display
This allows using context aware API directly.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2718>
