backends: Map left-handed in tablet "touchpads" together with the tablet

Avoid those devices when applying the configuration to touchpads, and ensure
the devices grouped with the tablet are all synchronized wrt the left handed
setting.

Closes: https://gitlab.gnome.org/GNOME/mutter/issues/562

src/backends/meta-input-settings.c

@@ -302,9 +302,30 @@ update_touchpad_left_handed (MetaInputSettings  *input_settings,
     }
   else
     {
-      settings_set_bool_setting (input_settings, CLUTTER_TOUCHPAD_DEVICE,
-                                 input_settings_class->set_left_handed,
-                                 enabled);
+      GSList *devices, *l;
+
+      devices = meta_input_settings_get_devices (input_settings,
+                                                 CLUTTER_TOUCHPAD_DEVICE);
+
+      for (l = devices; l; l = l->next)
+        {
+#ifdef HAVE_LIBWACOM
+          WacomDevice *wacom_device;
+
+          wacom_device = meta_input_settings_get_tablet_wacom_device (input_settings,
+                                                                      device);
+          /* "Touchpads" with a wacom description will be rotated along their tablets */
+          if (wacom_device &&
+              (libwacom_get_integration_flags (wacom_device) != WACOM_DEVICE_INTEGRATED_NONE))
+            continue;
+#endif
+
+          settings_device_set_bool_setting (input_settings, l->data,
+                                            input_settings_class->set_left_handed,
+                                            enabled);
+        }
+
+      g_slist_free (devices);
     }
 }
 
@@ -1068,8 +1089,12 @@ update_tablet_left_handed (MetaInputSettings  *input_settings,
                            ClutterInputDevice *device)
 {
   MetaInputSettingsClass *input_settings_class;
+  MetaInputSettingsPrivate *priv;
+  const GSList *devices, *l;
   gboolean enabled;
 
+  priv = meta_input_settings_get_instance_private (input_settings);
+
   if (clutter_input_device_get_device_type (device) != CLUTTER_TABLET_DEVICE &&
       clutter_input_device_get_device_type (device) != CLUTTER_PEN_DEVICE &&
       clutter_input_device_get_device_type (device) != CLUTTER_ERASER_DEVICE &&
@@ -1091,10 +1116,18 @@ update_tablet_left_handed (MetaInputSettings  *input_settings,
 
   input_settings_class = META_INPUT_SETTINGS_GET_CLASS (input_settings);
   enabled = g_settings_get_boolean (settings, "left-handed");
+  devices = clutter_device_manager_peek_devices (priv->device_manager);
 
-  settings_device_set_bool_setting (input_settings, device,
-                                    input_settings_class->set_left_handed,
-                                    enabled);
+  for (l = devices; l; l = l->next)
+    {
+      if (l->data != device &&
+          !clutter_input_device_is_grouped (device, l->data))
+        continue;
+
+      settings_device_set_bool_setting (input_settings, l->data,
+                                        input_settings_class->set_left_handed,
+                                        enabled);
+    }
 }
 
 static void