GF shell: change parse & linearize to obtain useful results from p|l and l|p in more cases

These changes are inspired by the gf -cshell implementation of these commands.

The output of the linearize command has been changed to remove superfluous
blank lines and commas, and deliver the result as a list of strings instead of
a single multi-line string. This makes it possible to use -all and pipe the
results to the parse command. This also means that with -treebank -all,
the language tag will be repeated for each result from the same language.

The parse command, when trying to parse with more than one language, would
"forget" other results after a failed parse, and thus not send all
successful parses through the pipe. For example, if English is not the first
language in the grammar,

    p "hello" | l

would output nothing, instead of translations of "hello" to all languages,
forcing the user to write

   p -lang=Eng "hello" | l

instead, to get the expected result. The cause of this behaviour was in the
function fromParse, which was rather messy, so I assume it is not intentional,
but the result of a programming mistake at some point.

The fromParse function has now been refactored from a big recursive function
into 

    fromParse opts = foldr (joinPiped . fromParse1 opts) void

where the helper functions fromParse1 deals with a single parse result and
joinPiped combines multiple parse results.

src/compiler/GF/Data/Operations.hs

@@ -38,7 +38,7 @@ module GF.Data.Operations (
                    tree2list,
  
 		   -- ** Printing
-		   indent, (+++), (++-), (++++), (+++++),
+		   indent, (+++), (++-), (++++), (+++-), (+++++),
 		   prUpper, prReplicate, prTList, prQuotedString, prParenth, prCurly, 
 		   prBracket, prArgList, prSemicList, prCurlyList, restoreEscapes,
 		   numberedParagraphs, prConjList, prIfEmpty, wrapLines,
@@ -160,13 +160,20 @@ tree2list = Map.toList
 indent :: Int -> String -> String
 indent i s = replicate i ' ' ++ s
 
-(+++), (++-), (++++), (+++++) :: String -> String -> String
+(+++), (++-), (++++), (+++-), (+++++) :: String -> String -> String
 a +++ b   = a ++ " "    ++ b
+
 a ++- ""  = a 
 a ++- b   = a +++ b
+
 a ++++ b  = a ++ "\n"   ++ b
+
+a +++- "" = a
+a +++- b  = a ++++ b
+
 a +++++ b = a ++ "\n\n" ++ b
 
+
 prUpper :: String -> String
 prUpper s = s1 ++ s2' where
  (s1,s2) = span isSpace s