babl-cache: store reference fishes in the cache

babl-cache: store reference fishes in the cache

When we only have a reference fish for a given conversion, and not
a path fish, store a corresponding entry in the cache, marked by a
"[reference]" tag in the fish's attributes line, and containing no
conversion list.  When reloading the cache, make sure a reference
fish is created for the conversion without trying to construct a
path fish, so that we don't waste time trying to search for a
conversion path.

Note that we still show a "missing fast path" warning, even for
cached reference fishes.