clear observables

src/Gizmos/babylon.positionGizmo.ts

@@ -97,6 +97,8 @@ module BABYLON {
             this.xGizmo.dispose();
             this.yGizmo.dispose();
             this.zGizmo.dispose();
+            this.onDragStartObservable.clear();
+            this.onDragEndObservable.clear();
         }
 
         /**

src/Gizmos/babylon.rotationGizmo.ts

@@ -98,6 +98,8 @@ module BABYLON {
             this.xGizmo.dispose();
             this.yGizmo.dispose();
             this.zGizmo.dispose();
+            this.onDragStartObservable.clear();
+            this.onDragEndObservable.clear();
         }
 
         /**

src/Gizmos/babylon.scaleGizmo.ts

@@ -121,6 +121,8 @@ module BABYLON {
             this.yGizmo.dispose();
             this.zGizmo.dispose();
             this.uniformScaleGizmo.dispose();
+            this.onDragStartObservable.clear();
+            this.onDragEndObservable.clear();
         }
     }
 }