Refactor `everyvoice demo` code to the submodules

[roedoejet]

See @joanise's comment:

"Is there any code at all that is shared between the styletts2 demo and the fs2+hfgl one? As I scan, it seems like it's completely separate code, and in that case I would think that this should be placed in styletts2/cli/demo.py instead of here.

That would also facilitate adding it to the styletts2 CLI as styletts2 demo, actually."

Originally posted by @joanise in #804 (comment)

[joanise]

Good idea to move this to an issue/future PR.