function EventHelper() {
  this._removalFunctions = [];
}
A convenience object that simplifies the common pattern of attaching event listeners
to several events, then removing all those listeners at once later, for example, in
a destroy method.

@alias EventHelper
@constructor


@example
const helper = new Cesium.EventHelper();

helper.add(someObject.event, listener1, this);
helper.add(otherObject.event, listener2, this);

// later...
helper.removeAll();

@see Event