Implement `dictionary->xliff`, `xliff->dictionary` (or equivalents) 

Haven't done any research yet on this myself, but it'd be nice (and hopefully pretty straight-forward) to provide some basic tools for translating Tower dictionaries to & from an industry-standard format.

This way we get things like translation-change-tracking (and a bunch of other useful stuff) for free, and it'll be easier to interface with professional translators.
