Trigger impress:steprefresh for every onStepEnter ( goto() )

Unlike impress:stepenter, we emit impress:steprefresh event also
when the "entered" step is the current step. This allows plugins
to reload or redraw objects if needed.

(Note that resize plugin already calls goto() on the active element
for similar purposes when it sees a window resize event. Emitting
impress:steprefresh allows other plugins to join in such a refresh,
and also others can call goto() if a refresh is needed.)

js/impress.js

@@ -225,11 +225,15 @@
         // `onStepEnter` is called whenever the step element is entered
         // but the event is triggered only if the step is different than
         // last entered step.
+        // We sometimes call `goto`, and therefore `onStepEnter`, just to redraw a step, such as
+        // after screen resize. In this case - more precisely, in any case - we trigger a
+        // `impress:steprefresh` event.
         var onStepEnter = function( step ) {
             if ( lastEntered !== step ) {
                 lib.util.triggerEvent( step, "impress:stepenter" );
                 lastEntered = step;
             }
+            lib.util.triggerEvent( step, "impress:steprefresh" );
         };
 
         // `onStepLeave` is called whenever the currentStep element is left