Refactor events to use Symfony Event Dispatcher + new event naming

[rhukster]

We're are moving to a more powerful event dispatcher system for our eventing. As part of this move we are also going to be changing the names of our events to the following format

on<Object><Action>

This will make things a bit tidier and should scale better with more plugins.

[mahagr]

@rhukster already implemented this one. Closing it.