backend: Set up and use ownership chains

This means objects have an owner, where the chain eventually always
leads to a MetaContext. This also means that all objects can find their
way to other object instances via the chain, instead of scattered global
singletons.

This is a squashed commit originally containing the following:

cursor-tracker: Don't get backend from singleton

idle-manager: Don't get backend from singleton

input-device: Pass pointer to backend during construction

The backend is needed during construction to get the wacom database.

input-mapper: Pass backend when constructing

monitor: Don't get backend from singleton

monitor-manager: Get backend directly from monitor manager

remote: Get backend from manager class

For the remote desktop and screen cast implementations, replace getting
the backend from singletons with getting it via the manager classes.

launcher: Pass backend during construction

device-pool: Pass backend during construction

Instead of passing the (maybe null) launcher, pass the backend, and get
the launcher from there. That way we always have a way to some known
context from the device pool.

drm-buffer/gbm: Get backend via device pool

cursor-renderer: Get backend directly from renderer

input-device: Get backend getter

input-settings: Add backend construct property and getter

input-settings/x11: Don't get backend from singleton

renderer: Get backend from renderer itself

seat-impl: Add backend getter

seat/native: Get backend from instance struct

stage-impl: Get backend from stage impl itself

x11/xkb-a11y: Don't get backend from singleton

backend/x11/nested: Don't get Wayland compositor from singleton

crtc: Add backend property

Adding a link to the GPU isn't enough; the virtual CRTCs of virtual
monitors doesn't have one.

cursor-tracker: Don't get display from singleton

remote: Don't get display from singleton

seat: Don't get display from singleton

backend/x11: Don't get display from singleton

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2718>

src/backends/native/meta-input-settings-native.c

@@ -831,6 +831,7 @@ MetaInputSettings *
 meta_input_settings_native_new_in_impl (MetaSeatImpl *seat_impl)
 {
   return g_object_new (META_TYPE_INPUT_SETTINGS_NATIVE,
+                       "backend", meta_seat_impl_get_backend (seat_impl),
                        "seat-impl", seat_impl,
                        NULL);
 }