PGF service & minibar: only show dependency diagrams if the labels are known

+ The PGF service now reads and caches dependency label configuration files.
+ The grammar info returned by command=grammar has a new boolean field 
  'hasDependencyLabels' to indicate if dependency labels were found for
  the grammar. Also, command=deptree will now fail if no labels are present.
+ The minibar only shows word dependency trees if labels are present.
+ Also changed the type of getDepLabels from [String] -> Labels to
  String -> Labels, since all uses were in the form "getDepLabels . lines".

src/compiler/GF/Server.hs

@@ -174,7 +174,7 @@ handle logLn documentroot state0 cache execute1 stateVar
                (_  ,_             ,".pgf") -> do --debug $ "PGF service: "++path
                                                  wrapCGI $ PS.cgiMain' cache path
                (dir,"grammars.cgi",_     ) -> grammarList dir (decoded qs)
-               (dir  ,"exb.fcgi"  ,_    ) -> wrapCGI $ ES.cgiMain' root dir (fst cache)
+               (dir  ,"exb.fcgi"  ,_    ) -> wrapCGI $ ES.cgiMain' root dir (PS.pgfCache cache)
                _ -> serveStaticFile rpath path
              where path = translatePath rpath
            _ -> return $ resp400 upath