Add a plugin / layout mode that makes viewer.js look like iframe viewer

It would be nice if the out-of-the-box viewer.js code had a way to toggle a bottom toolbar with the same functionality as the iframe-mode file viewer.

Start by comparing the out-of-the-box viewer.js experience with the iframe-mode file viewer, focusing on the bottom toolbar and its functionality. Define the plugin or layout toggle boundary and confirm that the completed mode exposes the same toolbar capabilities without changing the default viewer behavior.

Written by the indexing model from the issue text.