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.

Config/Files.hs

@@ -33,7 +33,7 @@ modifyAutoStartFile func = do
 	let dirs' = nubBy equalFilePath $ func dirs
 	when (dirs' /= dirs) $ do
 		f <- autoStartFile
-		createDirectoryIfMissing True (parentDir f)
+		createDirectoryIfMissing True (takeDirectory f)
 		viaTmp writeFile f $ unlines dirs'
 
 {- Adds a directory to the autostart file. If the directory is already