 . add better quality setting for each format.
 * HIG compliant interface
 * progress dialog
 . handle error when overwriting a file
 * autotools, l12n
 * handle exceptions even when they happen in callbacks
 * give time estimates for progress bar
 * choose dir, add recursively input files that need to be converted
 * bug: the program's options aren't printed with --help
 * bug: the dialog can only be showed once
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