Put all chord signatures into a single array

This simplifies the code quite a bit. Instead of a big switch
statement based on number of notes in the chord, with each case a
nearly-identical for loop, we can have one single for loop for all
signatures.

More importantly, this means that after the for loop has matched a
signature, we can have a single variable pointing to the matching
signature. That will be important for using the signature to guide in
spelling the chord correctly, which we hope to do soon.