Bug 441437 - Enable folder lookup without directly using RDF, r=rkent

While the folder lookup here still uses RDF internally to find folders in the
first place, this allows us to both hide RDF's idiosyncracies from most users
and replace the lookup mechanism with a more principled variant later without
needing to rewrite many callsites.

(CLOSED TREE for SeaMonkey's manifest)

--HG--
extra : amend_source : 21c4338dc813c21500aae9d6cc1a44332bcba77f