Add debug_dump_scene()

Dump view_tree and osd_tree on calling debug-action. Example bind:

<keybind key="A-F12">
  <action name="Debug"></action>
</keybind>

src/meson.build

@@ -3,6 +3,7 @@ labwc_sources = files(
   'buffer.c',
   'cursor.c',
   'damage.c',
+  'debug.c',
   'desktop.c',
   'foreign.c',
   'interactive.c',