Vietnamese translation

A complete translation into Vietnamese. For the most part, the translations are all consistent with existing translations of Potlatch, Potlatch 2, and the Rails port, except where iD intentionally introduces new terms like “line”. I took some liberties in the mode descriptions to make them read better. For example, I removed “highway” since *đường cao tốc* is somewhat redundant to the word for “road”, *con đường*, and I substituted “canal” for “waterway”. I translated some of the keyboard shortcuts, but not all, since some of them have little to do with the function’s name, like [V], which looks like the head of a directional arrow.