FLUID-5506: Support for first part of FLUID-5506 functionality - stacking for namespaced events.

Rationalised fluid.makeEventFirer API - removed deprecated fluid.event.getEventFirer, removed "unicast" option, moved to full "options" model even though it increases our object count per event by 1. This can probably be removed by arg-ising all of the event firer impl and removing its giant closure. Converted AriaLabeller to a genuine Infusion component, eliminating the last client of "unicast" in the framework.