common: add node-type.c/h (renaming ssd_part_type to lab_node_type)

ssd_part_type contains several node types that are not actually part of
server-side decorations (ROOT, MENU, OSD, etc.)

Rename it accordingly and move it to a common location, along with some
related conversion/comparison functions.

src/common/meson.build

@@ -12,6 +12,7 @@ labwc_sources += files(
   'match.c',
   'mem.c',
   'nodename.c',
+  'node-type.c',
   'parse-bool.c',
   'parse-double.c',
   'scene-helpers.c',