A-Frame plugin: introduce the arfinished event

docs/api/plugin-aframe.md

 **Details**
 
 * `ar: object`. A reference to the [ar](#ar) system.
+
+### arfinished
+
+The AR session has ended.
+
+*Since:* 0.4.2
+
+**Details**
+
+* `ar: object`. A reference to the [ar](#ar) system.

plugins/aframe-with-encantar.js

             this.viewer = null;
             this.frame = null;
             this.pointers.length = 0;
+
+            scene.emit('arfinished', { ar: this });
         });
 
         session.viewport.addEventListener('resize', () => {