feat(api): add notification for when filmstrip gets toggled (#3974)

modules/API/API.js

@@ -551,6 +551,21 @@ class API {
         this._sendEvent({ name: 'feedback-prompt-displayed' });
     }
 
+    /**
+     * Notify external application (if API is enabled) that the display
+     * configuration of the filmstrip has been changed.
+     *
+     * @param {boolean} visible - Whether or not the filmstrip has been set to
+     * be displayed or hidden.
+     * @returns {void}
+     */
+    notifyFilmstripDisplayChanged(visible: boolean) {
+        this._sendEvent({
+            name: 'filmstrip-display-changed',
+            visible
+        });
+    }
+
     /**
      * Notify external application (if API is enabled) that the screen sharing
      * has been turned on/off.