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/tests/headless-start-test.c

@@ -102,6 +102,7 @@ meta_test_headless_monitor_connect (void)
   gpu = META_GPU (meta_backend_get_gpus (meta_get_backend ())->data);
   crtc = g_object_new (META_TYPE_CRTC_TEST,
                        "id", (uint64_t) 1,
+                       "backend", backend,
                        "gpu", gpu,
                        NULL);
   test_setup->crtcs = g_list_append (NULL, crtc);

src/tests/meta-monitor-test-utils.c

@@ -664,6 +664,7 @@ meta_create_monitor_test_setup (MetaBackend          *backend,
 
       crtc = g_object_new (META_TYPE_CRTC_TEST,
                            "id", (uint64_t) i + 1,
+                           "backend", backend,
                            "gpu", meta_test_get_gpu (backend),
                            NULL);
       if (setup->crtcs[i].disable_gamma_lut)