Introduce Trackable.tracker

docs/api/trackable.md

 # Trackable
 
-A Trackable is an interface that represents something that is tracked by a [Tracker](tracker.md).
+A Trackable is an interface that represents something that is tracked by a [Tracker](tracker.md).
+
+## Properties
+
+### tracker
+
+`trackable.tracker: Tracker, read-only`
+
+A reference to the [Tracker](tracker.md) that generated this trackable.
+
+*Since:* 0.4.0

src/trackers/image-tracker/states/tracking.ts

             // the trackable object
             const trackable: TrackableImage = {
                 pose: pose,
-                referenceImage: referenceImage
+                referenceImage: referenceImage,
+                tracker: this._imageTracker
             };
 
             // the result generated by the image tracker

src/trackers/pointer-tracker/pointer-tracker.ts

 
             // we create new trackable instances on each frame;
             // these will be exported and consumed by the user
-            this._newPointers.set(id, { id, phase, position, deltaPosition, initialPosition, velocity, duration, movementDuration, movementLength, isPrimary, kind });
+            this._newPointers.set(id, {
+                id, phase,
+                position, deltaPosition, initialPosition,
+                velocity, duration,
+                movementDuration, movementLength,
+                isPrimary, kind,
+                tracker: this
+            });
 
         }
 

src/trackers/tracker.ts

  */
 export interface Trackable
 {
+    /** the tracker that generated this trackable */
+    readonly tracker: Tracker;
 }
 
 /**