Made it clear that using `play()` on a paused sound resumes it

Also reduces a single line of repeated code
Merge two `FlxSound::_fadeTotal` variables

Since you can't fade out and fade in at the same time, having two different
variables handling the total amount of time to fade was unecessary.

Merges `_fadeInTotal` and `_fadeOutTotal` into `_fadeTotal`.
`FlxSound` Reset fade timers when a sound stops

All fading and all timers reset once a sound has stopped (or is paused)
`FlxSound::update()` amplitude cleanup

Remove unecessary check if `_channel` is `null` (it won't ever be when `update()` runs unless the developer overrides that behavior)

Reset the `amplitude` variables if the sound is muted (rather than leave them at their previous value)
Add `FlxSound::playing` field to avoid duplicate code

I realize we aren't supposed to add, remove, or modify public members in this release,
but this one was minor, and won't break any existing code.
Restrict `FlxSound` fading based on sound state

* Only able to `fadeIn()` if the sound is stopped or paused
* Only able to `fadeOut()` if the sound is playing
Add `resetFading` parameter to `FlxSound::cleanup()`

This allows you to resume fading in/out after resuming a paused sound.
Add FlxTween

A simple class for tweening a value from one to another.

Note that this class does not use the "global" time to progress
the tween, but must manually be incremented with the `progress`
property.
Use FlxTween in FlxSound

Isn't that much clearer? :)
Correct multiple typos in FlxTween and FlxSound

That's what I get for using a text editor rather than an IDE.
FlxSound looped sounds resume properly

If a looped sound is resumed, it only plays once from the "pause"
point, THEN it resets and plays 9999 loops from the beginning.

Fixes https://github.com/FlixelCommunity/flixel/issues/120
[FlxSound] Clean up `destroy()` code

Made sure to properly destroy all objects, though, a few objects seem to be destroyed more than once.
Removes all event listeners before destruction.

Also cleans up some of the ASDoc or adds it where needed.
Merge remote-tracking branch 'IQAndreas/fix-flxsound-fade' into dev

Resolved conflicts:
	org/flixel/FlxSound.as