Adds onFirstListener and onNoListener callbacks

[DomBlack]

These callbacks allow anything extending Backbones.Events to be
notified when something starts listening to it, and when nothing
is left listening to it.

Because of internalOn this is hard to implement in user land.

Use Case

The use case for this is if you had a Model presenting the output of
an expensive continuous calculation (say real time Stock Prices or
a timer).

The onFirstListener function can be used to start your expensive
calculation, as you only care about it while the something is listening
to changes in it.

When that item stops listening to changes in it, onNoListeners will
trigger, allowing you to clean up your code (unsubscribe the stock price,
or stop the timer or even stop listening to another Event).

[akre54]

Thanks but no thanks. Feel free to do this in your own project if you need it

[DomBlack]

NP. I have but I ended up having to do this in Backbone because of #3615 once that PR is merged, this can be done in user land.