Making Backend trait public

[saona-raimundo]

Hi! I know this could be a major rework and this is not a feature request, but a question if you would be up to review such PR.

I wanted to ask about the possibility of making the Backend trait public.

Pros

People could implement their own backend. So the following things would be possible;

• If I am not mistaken, espeakng-sys could implement a new backend... or any other ffi to TTS systems.
• People could implement their own in-house backend (for example: based on a dictionary, play some audio)

Cons

The cost of changing the API?

• Tts would take anything that implements the trait (for example Box<dyn Backend>), instead of Backends.

[ndarilek]

Sorry I haven't gotten to this sooner.

If you could make a PR for it then I'd be willing to review. I'm a bit concerned about the complexity of the various IDs and such. An enum was the easiest way to do those, but it can't be extended to support custom backends. So I guess it'd need to be a trait, or we'd need to rework the callbacks global to accept something that could be extended. It isn't a priority for me since it isn't something I need, so I probably won't get to it for a while or ever. But I'm happy to check someone else's work.