FlxSound and FlxG.sound: Further rework load methods

[Geokureli]

On top of #3479, I think we need to simplify loading and playing sounds, sound.load should only load the sound, it doesn't need a setter for every field. Likewise FlxG.sound.load doesn't need to behave like loadButAlsoPlay() we have play() for that.

@ACrazyTown looping you in as I'll be modifying some of your 6.2.0 changes

[ACrazyTown]

Sounds good. Semi-related, but while we're already a bunch of changes in FlxSound, maybe it would nice to also revisit #3272 for 6.2.0?

Also, I'm assuming you opened this on the haxeflixel.com repo by accident

[Geokureli]

Posted the old one in the wrong repo