`dtab`: better escaped character support about onyx

[mtolly]

While we're at it, do some testing with non-ASCII characters. Latin-1 is supposed to work correctly but I have seen it get corrupted in e.g. Grinnz's Ænima.

[mtolly]

I think the only remaining thing here is to turn " into ' for Magma (it's supported by the actual game, I think, but Magma disallows it for some reason)

[mtolly]

On second thought, modifying the artist depending on the backend would be unwise -- then you could end up with 2 different versions of the artist name in-game. I think we'll just not support the double quote, like Magma. After a final bug fix in f4cf189 I think this is finished. (Non-ASCII encoding is also correct; Ænima/Ænema shows up fine.)