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/meta-input-settings.c

@@ -39,6 +39,17 @@
 #include "core/display-private.h"
 #include "meta/util.h"
 
+enum
+{
+  PROP_0,
+
+  PROP_BACKEND,
+
+  N_PROPS
+};
+
+static GParamSpec *props[N_PROPS] = { 0 };
+
 static GQuark quark_tool_settings = 0;
 
 typedef struct _MetaInputSettingsPrivate MetaInputSettingsPrivate;
@@ -66,6 +77,8 @@ struct _DeviceMappingInfo
 
 struct _MetaInputSettingsPrivate
 {
+  MetaBackend *backend;
+
   ClutterSeat *seat;
   gulong monitors_changed_id;
 
@@ -1711,6 +1724,26 @@ meta_input_settings_constructed (GObject *object)
   load_keyboard_a11y_settings (input_settings);
 }
 
+static void
+meta_input_settings_set_property (GObject      *object,
+                                  guint         prop_id,
+                                  const GValue *value,
+                                  GParamSpec   *pspec)
+{
+  MetaInputSettings *settings = META_INPUT_SETTINGS (object);
+  MetaInputSettingsPrivate *priv =
+    meta_input_settings_get_instance_private (settings);
+
+  switch (prop_id)
+    {
+    case PROP_BACKEND:
+      priv->backend = g_value_get_object (value);
+      break;
+    default:
+      G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
+    }
+}
+
 static void
 meta_input_settings_class_init (MetaInputSettingsClass *klass)
 {
@@ -1718,6 +1751,17 @@ meta_input_settings_class_init (MetaInputSettingsClass *klass)
 
   object_class->dispose = meta_input_settings_dispose;
   object_class->constructed = meta_input_settings_constructed;
+  object_class->set_property = meta_input_settings_set_property;
+
+  props[PROP_BACKEND] =
+    g_param_spec_object ("backend",
+                         "backend",
+                         "MetaBackend",
+                         META_TYPE_BACKEND,
+                         G_PARAM_WRITABLE |
+                         G_PARAM_CONSTRUCT_ONLY |
+                         G_PARAM_STATIC_STRINGS);
+  g_object_class_install_properties (object_class, N_PROPS, props);
 
   quark_tool_settings =
     g_quark_from_static_string ("meta-input-settings-tool-settings");
@@ -1864,3 +1908,12 @@ meta_input_settings_get_kbd_a11y_settings (MetaInputSettings   *input_settings,
 
   *a11y_settings = priv->kbd_a11y_settings;
 }
+
+MetaBackend *
+meta_input_settings_get_backend (MetaInputSettings *settings)
+{
+  MetaInputSettingsPrivate *priv =
+    meta_input_settings_get_instance_private (settings);
+
+  return priv->backend;
+}