CSound support

[thibaudk]

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Both as a backend, to write csond plugins with avendish, and as a wrapper around csound to package it inside an avendish plugin, making the use of csound available from inside of score, and all other supported backend.

[jcelerier]

for making plugins for csound it makes a lot of sense but for the reverse I don't know if it wouldn't be easier to just use the normal, dynamic score API

[jcelerier]

also relevant: https://github.com/ComposersDesktop/CDP8