made parentDir return a Maybe FilePath; removed most uses of it

parentDir is less safe than takeDirectory, especially when working
with relative FilePaths. It's really only useful in loops that
want to terminate at /

This commit was sponsored by Audric SCHILTKNECHT.

Assistant/Install/Menu.hs

@@ -38,7 +38,7 @@ fdoDesktopMenu command = genDesktopEntry
 
 installIcon :: FilePath -> FilePath -> IO ()
 installIcon src dest = do
-	createDirectoryIfMissing True (parentDir dest)
+	createDirectoryIfMissing True (takeDirectory dest)
 	withBinaryFile src ReadMode $ \hin ->
 		withBinaryFile dest WriteMode $ \hout ->
 			hGetContents hin >>= hPutStr hout