Some general improvements

* Some formatting in FlxDestroyUtil
* Destroying arrays no longer does a horribly slow way of removing all the elements (The clearing probably isn't actually needed at all, but until I convince people of that, at least this way is better)
* Moved the FlxSignal macro into it's own class. This was recommended to me by folks in the haxe irc. It makes importing much nicer as you don't need to have compiler conditionals around imports in any class that imports this.