mark events as attended

this adds the posibility of declaring an event listener as:

   bool mousePressed(ofMouseEventArgs & mouse){
       if(....){
          return true;
       }else{
          return false;
       }
   }

returning true will mark the event as attended and won't be passed to any other listener.

It also adds the possibility of adding a listener with priority so you can control the order in which listeners are called. And there's 3 default priorities:

	OF_EVENT_ORDER_BEFORE_APP=0,
	OF_EVENT_ORDER_APP=100,
	OF_EVENT_ORDER_AFTER_APP=200

old event listeners that don't specify priority are added as AFTER_APP, and other priorities can be specified using numbers:

      ofAddListener(event,this,&MyClass::listener,priority);