Add cancelTweensOf and completeTweensOf

[Geokureli]

Added the ability to cancel or complete specific tweens without needing a reference to the tweens themselves. Useful when multiple things are applying tweens to the same object and want to remain agnostic of each other.

Usage:

FlxTween.tween(mySprite, { color: FlxColor.BLACK }, 1.0);

//... elsewhere ...
FlxTween.cancelTweensOf(mySprite);

// or, to only cancel color tweens on mySprite
FlxTween.cancelTweensOf(mySprite, "color");

also works with tweens that are not VarTweens by treating them as VarTweens based on the properties they change, such as:
AngleTween: "angle":
ColorTween: "color"
Motion (and all derivatives): "x" and "y"

completeTweensOf works identical, but finishes the tween and calls onComplete

Just like with VarTweens, properties passed in can be dot paths to denote properties of children, this way any field passed in to a var tween can be used to cancel or complete the tween
Example:

// either
FlxTween.tween(mySprite, { "child.color": FlxColor.BLACK }, 1.0);
// or
FlxTween.color(mySprite.child, 1.0, FlxColor.BLACK, FlxColor.RED);

// can be cancelled via:
FlxTween.cancelTweensOf(mySprite, ["child.color"]);
// or
FlxTween.cancelTweensOf(mySprite.child, ["color"]);

fixes #2180

[Gama11]

Are any of the changes in VarTween changes besides the isTweenOf() override related to this PR? It's not clear to me why they were made.

[Geokureli]

It calls initializeVars earlier than before so that we can search _propertyInfos in isTweenOf() while that tween is in the delay phase. originally, this initialization was delayed until the tween starts so that chaining works correctly, but that delay is only needed for the startValues.

If you think I need to explain this in code, let me know