SimpleEventDispatcher should use protected visibility instead of private

[swiftmailer]

I recently encountered a situation where I had to ensure that registered listener plugins were called in a specific sequence. To do this I tried creating a class which inherits from Swift_Events_SimpleEventDispatcher and which manages listener order based on a priority property.
However, in the Swift_Events_SimpleEventDispatcher class the listener fields (to be specific: the _listener array) have private visibility - is there a special reason why protected isn't used here?

Original creation date: 2010-12-09T13:45:04Z
Original reporter: martinl
Original ticket: http://swiftmailer.lighthouseapp.com/projects/21527/tickets/169