Add node-descriptor for wlr_scene_nodes

Support identification of wlr_scene_node role to enable simplification
of codebase including the avoidance of iterating over lists of
layer-surface, menuitems, and so on.

Use node-descriptors for xdg toplevels and popups

src/meson.build

@@ -11,6 +11,7 @@ labwc_sources = files(
   'key-state.c',
   'layers.c',
   'main.c',
+  'node-descriptor.c',
   'osd.c',
   'output.c',
   'resistance.c',