FlxGroup: implement revive()

[shinji-yoshida]

FlxTypedGroup has method kill() that kills all members in group, but revive() did not revive them.
I have implemented revive() so that all members to be revived.
Please merge.

[Gama11]

I think this might cause issues with FlxEmitter that calls revive() in start().

[shinji-yoshida]

Thank you for pointing out.
I have added reviveOnlyGroup() to FlxTypedGroup and use it in FlxEmitter#start().

[Gama11]

I'm not really a fan of this solution, tbh, but I can't really think of a better one either since haxe has no function overloading...

[shinji-yoshida]

I pushed another solution. What do you think?

[Gama11]

To be honest, I think it's quite a mess and way too implicit (I would prefer an explicit solution like adding a reviveMembers param to the function, but since you can't change the function signature, that's out of the question). Something this simple shouldn't require this much code.

Now, I do agree that revive() should probably revive all its members to be consistent with kill() - anything else seems counter-intuitive. I haven't checked, but it's probably fine if we replace the revive() call in FlxEmitter by exists = true?

[shinji-yoshida]

It seems that I had clung to calling revive() too much.
Replacing revive() by exists = true is much better!

I have replaced revive() by exists = true and rebased to latest dev.
Please check it.