Fix iteration bug in FlxSignal

[kevinresol]

There would be array iteration bug if the listener is removed while the signal is processing the array of listeners inside dispatch() (no matter the remove is invoked by dispatchOnce property or by user code through the remove() function)

[Gama11]

What exactly is the bug / got an example that reproduces it?

[kevinresol]

In short, the bug appears when you remove() a listener while the signal is dispatch()ing. It is just the classic "remove item while iterating" bug.

var signal = new FlxSignal();
var listener1 = function() trace("L1");
var listener2 = function() trace("L2");
signal.addOnce(listener1);
signal.add(listener2);
signal.dispatch(); // trace: L1 (L1 triggered but not L2 due to the bug)
signal.dispatch(); // trace: L2 (only trigger L2 as L1 already removed)