Replace `FlxSound.onComplete` with `FlxSound.onFinish`

[DetectiveBaldi]

Adjusts FlxSound to comply with new changes in flixel involving changing a bunch of single callbacks to use FlxSignals instead.

[DetectiveBaldi]

I also wanna make sure init() can't be called multiple times, I.E. via recycling or something. if it can then we would need to find a way to remove the listener added there, but not any added externally

I did some more testing with recycling. I noticed that the callbacks would not be cleared if you killed a sound, and it got recycled. That is fixed with commits I'm pushing now, however I do want to note that some of the line ordering may look a bit inconsistent due to when FlxSound calls certain functions, which might be a problem

Also regarding making onFinish a final: It requires me to assign the variable in the constructor compared to in the reset method like all other variables in the class, might not be important, just something to considered

[Geokureli]

Thanks, won't be able to test this for a short while, though