babl: cache state between babl runs

babl: cache state between babl runs

Store the contents of discovered babl_paths between runs, for now in
/tmp/babl.db , should move to a per-user dir to make it survive reboots.

The code should also check a versioning string at the start and bail if there
is any mismatch... since the data should be reconstructable; and more reliable
when reconstructed than a possibly old version.