core: Setup and use ownership chains

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>

src/core/startup-notification-private.h

@@ -49,4 +49,6 @@ MetaStartupSequence *
          meta_startup_notification_lookup_sequence (MetaStartupNotification *sn,
                                                     const gchar             *id);
 
+MetaDisplay * meta_startup_sequence_get_display (MetaStartupSequence *seq);
+
 #endif /* META_STARTUP_NOTIFICATION_PRIVATE_H */