wayland: Look for default cursor theme in XDG directories

Currently it's looked up only in /usr/share what is a problem for non-FHS distros like NixOS

gdk/wayland/gdkdisplay-wayland.c

@@ -1130,6 +1130,9 @@ get_cursor_theme (GdkWaylandDisplay *display_wayland,
         return theme;
     }
 
+  if (strcmp (name, "default") != 0)
+    return get_cursor_theme (display_wayland, "default", size);
+
   /* This may fall back to builtin cursors */
   return wl_cursor_theme_create ("/usr/share/icons/default/cursors", size, display_wayland->shm);
 }