/** 
    \class EventSender
    
    Sender of events of type EvType. All public methods are static since there is 
    only one instance of the sender. EventSender maintains a registry 
    of listeners who want to receive the events it sends. 
    
    Usage: 
    - No instantiation needed (actually, none is possible)
    - Call add() and remove() to add/remove a listener to/from the
      registry. Only listeners in the registry receive events.  If those
      two methods are called while a send() is already in progress
      (typically from within your Listener<EvType>::processEvent()), 
      the registrations/removals are queued and are enacted only 
      once all listeners have received the event. This ensures the 
      integrity of the registry during a send().
    - Call send() on an event to send it to all registered listeners. 
    - Call isSending() to see whether or not EventSender<EvType> is currently
      doing a  send() call (typically used from within your
      Listener<EvType>::processEvent() to see whether sending another
      event  of type EvType is safe). 
    - Call getNumListeners() to see how many listeners are currently 
      registered. This number does not include listeners queued for
      registration or removal from registry.
    - Call getMinNumIgnored() to see how many listeners have ignored the 
      last event sent. If called during a send(), gives the current 
      number. Regardless, this number is a minimum since listeners are 
      not required to tell EventSender<EvType> that they are ignoring the received
      event. 
*/