General multi value support for MigrateFileField Handlers' arguments

Similar to #1534968: Multiple files, multiple descriptions and #1509750: Trying to map subfields (file field description) to a multi-value file field, I had trouble to import multiple images to a node's image field. In my case I need a different destination_dir for each single file. The current implementation only supports multiple values for 'alt' and 'title' of an imagefield. The same functionality could be provided for file fields in general by simply check on arrays in the arguments.

I'll provide a patch in the first comment (to get the issue number). I'd would be great someone could review that.