Use this._popupHandlersAdded to manage Marker popup event

This make the internal API more consistent with Path.

My use case is to disable event management in bindPopup (because I manage them myself).

Also, I've added `this` argument to `this.off` calls, otherwise the signature of the event would not be the same as the one added with `this.on`. I've thought it was a really minimal change to commit it, but if you prefer a separate PR or separate commit at least, it's fair, just ask :)

Thanks!