function ContextPad(canvas, elementRegistry, eventBus, scheduler) {

  this._canvas = canvas;
  this._elementRegistry = elementRegistry;
  this._eventBus = eventBus;
  this._scheduler = scheduler;

  this._current = null;

  this._init();
}
A context pad that displays element specific, contextual actions next
to a diagram element.

@param {Canvas} canvas
@param {ElementRegistry} elementRegistry
@param {EventBus} eventBus
@param {Scheduler} scheduler