Work on example-based grammar writing

+ Preparations to support more target languages. Tested with Swedish.
+ Don't enable example-based editing if there is less that two concrete
  languages in the grammar.
+ Don't show the "By example" button until the required lincat has been defined.
+ Remember the chosen language for examples.

src/example-based/ExampleService.hs

@@ -72,7 +72,9 @@ getLimit = maybe err return =<< readInput "limit"
   where err = throwCGIError 400 "Missing/bad limit" []
 
 
-readParsePGF cwd cache = liftIO $ readCache cache (cwd</>"ParseEngAbs.pgf")
+readParsePGF cwd cache =
+    do parsepgf <- getInp "parser"
+       liftIO $ readCache cache (cwd</>parsepgf)
 
 parseEnviron s = do state <- liftIO $ readIO s
                     return $ environ state