notify popup management: needs improving

[moloko]

Currently NotifyView handles the global "notify:close" and "notify:cancel" events as well as the events local to each notify view.

This is not ideal and has led to some slightly inconvenient code like this: https://github.com/adaptlearning/adapt_framework/blob/5a3de6952f016e70aacd986a9e6c71e0ebed462b/src/core/js/views/notifyView.js#L194-L198

Well, it's inconvenient to me anyway because I have a situation where, in my implementation for adaptlearning/adapt-contrib-spoor#245, it's possible to end up with the bookmarking popup appearing over the top of a spoor error message - which obviously isn't desirable. I can force the spoor error to appear on top, but the above code then prevents it from being it from being closed because it's not actually the top-most in that 'stack'...

It would be better to move the handlers for the global "notify:close" and "notify:cancel" events out of NotifyView and into Notify itself.

Some work will also be required on Adapt.a11y.popup to ensure that the popup 'layer' management still works even if the notify popups are closed out-of-order

[oliverfoster]

adaptlearning/adapt_framework#3029