Report number of pending promises on each emitter.emit()

[fjcalzado]

Instead of just notify a boolean whenever a change in the progress is made:

emitter.emit(progressEventId, true/false);

We could report the counter itself, which determines the number of pending promises. This can be done instead of the true/false flag:

emitter.emit(progressEventId, counter);

or providing backwards compatibility (extending the emit with an extra parameter):

emitter.emit(progressEventId, true/false, counter);

[fjcalzado]

This is fairly easy @brauliodiez , what do you think? It will allow us to add a number of pending operations in the spinner.

[brauliodiez]

I think it's time to go for a feature like this.

We have to balance wether to break the current api or try to maintain backward compatiblity.