JI-1352 Add event when h5p has finished initializing

This is useful if you want to hook into H5P functionality after the
init function has run, e.g. 'resize' event in instances

js/h5p.js

@@ -369,6 +369,10 @@ H5P.init = function (target) {
     $element.on('mousedown keydown keyup', function (event) {
       $element.toggleClass('using-mouse', event.type === 'mousedown');
     });
+
+    if (H5P.externalDispatcher) {
+      H5P.externalDispatcher.trigger('initialized');
+    }
   });
 
   // Insert H5Ps that should be in iframes.