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/Command/Commands.hs

@@ -551,7 +551,7 @@ pgfCommands = Map.fromList [
          let outp = valStrOpts "output" "dot" opts
          mlab <- case file of
            "" -> return Nothing
-           _  -> (Just . getDepLabels . lines) `fmap` restricted (readFile file)
+           _  -> (Just . getDepLabels) `fmap` restricted (readFile file)
          let lang = optLang pgf opts
          let grphs = map (graphvizDependencyTree outp debug mlab Nothing pgf lang) es
          if isOpt "conll2latex" opts
@@ -616,7 +616,7 @@ pgfCommands = Map.fromList [
          let depfile = valStrOpts "file" "" opts
          mlab <- case depfile of
            "" -> return Nothing
-           _  -> (Just . getDepLabels . lines) `fmap` restricted (readFile depfile)
+           _  -> (Just . getDepLabels) `fmap` restricted (readFile depfile)
          let grphs = map (graphvizParseTreeDep mlab pgf lang gvOptions) es
          if isFlag "view" opts || isFlag "format" opts
            then do