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/runtime/haskell/PGF.hs

@@ -131,7 +131,7 @@ module PGF(
            graphvizDefaults,
            conlls2latexDoc,
            -- extra:
-           getDepLabels,
+           Labels, getDepLabels,
  
            -- * Probabilities
            Probabilities,