Add a way to get an Audio Bus Effect's index.

[andrewharn]

This is how I handle the effects I want to add to an audio bus.

As you can see, I store the effects in a dictionary to keep an easy access to them. It's really convenient.

But uhh... there's no way to safely access them when it comes to things like removing them from the bus. An index is needed for that, and while yes it's completely fine when you have some effects where you're sure of their positions, what if you have many of them? What if one get removed and gets shifted like some elements in an array? The indexes could get confusing and lost really quickly.

So, here I am suggesting a little function to fix this problem! Something like this:

After all, even things like AnimationPlayer tracks have similar methods, why not Audio Buses' effects?

I hope this is enough to be a valid proposal, if I need to add anything else, let me know!