EventSequencer: New hooks for useful moments

ve.EventSequencer.js - Implement new types of event listener:
* onLoop( f ) to set a listener for the start of the event loop
* afterLoop( f ) to set a listener for the end of the event loop
* afterOne( f ) to set a one-time listener for after an event
* afterLoopOne( f ) to set a one-time listener for the end of the event loop

Change-Id: Ie388e0e9edcfccaa20e04c649a8b85d028ddde9c
Merge "EventSequencer: New hooks for useful moments"