Latin Extended-B changes

See comments in individual commits for details.

I have about 4 more rows to go before I get to the IPA characters.

Also, I haven't mentioned this in the commits, but I've been expanding the character classes in kerning class 0, e.g. grouping open O with O, florin with f, yogh with z, and so on. I haven't been documenting exactly which characters I added to which classes; maybe I should in future.