Add a 'boxdrag' event to ol.interaction.DragBox. Resolves #4563 .

src/ol/interaction/dragboxinteraction.js

@@ -29,6 +29,14 @@ ol.DragBoxEventType = {
    * @api stable
    */
   BOXSTART: 'boxstart',
+
+  /**
+   * Triggered on drag when box is active.
+   * @event ol.DragBoxEvent#boxdrag
+   * @api
+   */
+  BOXDRAG: 'boxdrag',
+
   /**
    * Triggered upon drag box end.
    * @event ol.DragBoxEvent#boxend
@@ -167,6 +175,9 @@ ol.interaction.DragBox.handleDragEvent_ = function(mapBrowserEvent) {
   }
 
   this.box_.setPixels(this.startPixel_, mapBrowserEvent.pixel);
+
+  this.dispatchEvent(new ol.DragBoxEvent(ol.DragBoxEventType.BOXDRAG,
+    mapBrowserEvent.coordinate, mapBrowserEvent));
 };