update-po: Keep POT's order of translations

[Blipz]

It would be nice (if not the default, as an option) to keep translations in the order they appear in the POT file when executing the update-po command.

This would help translators better understand the context (keep messages close to one another if they originate from the same source file) and make the diffs clearer (for instance, when a source message is changed in the code).