desktop: don't use LAB_NODE_LAYER_SUBSURFACE node type

Instead, set ctx.type = LAB_NODE_LAYER_SURFACE for both layer-surfaces
and layer-subsurfaces.

This patch preserves the existing behaviors:
- Pressing a subsurface of an on-demand layer-surface gives pointer
  focus to the subsurface, but gives keyboard focus to the parent
  layer-surface (related: a5fcbfaf).
- Pressing a subsurface of a layer-surface doesn't close a popup
  (related: a89bcc3c).

src/common/meson.build

@@ -17,7 +17,6 @@ labwc_sources += files(
   'parse-double.c',
   'scene-helpers.c',
   'set.c',
-  'surface-helpers.c',
   'spawn.c',
   'string-helpers.c',
   'xml.c',