Blend sample banks

[yaxu]

It would be good if we could specify multiple banks at once:

s("bd useme folkharp").bank("s_:breaks")

They're then if a sampleset exists in more than one, it's either filled from the first listed bank in order of priority, or combined into one big bank. Not sure what's best.

For this and in general it'd be great to failback to no bank, if a sampleset isn't present in the default bank.