[Feature] Getting the number of notified listeners back

[jjl]

Hello!

I've just built an RWLock based around a single atomic and now I'm trying to make an async version with eventlistener. For my purposes, simply knowing if any listeners were notified would suffice, but knowing the count seems a logical extension of that.

What do you think?

[jjl]

Actually it turns out I don't need this anymore, but I still think it's a good idea for at some point :)

[ghost]

Seems like a reasonable addition to me :)

[notgull]

Since we're making breaking changes anyways, it might be worthwhile to add this.