This PR introduces an updated builder interface that uses an iframe to display the layout you are editing for accurate previews at different device sizes. The responsive editing interface has also been updated to allow you to preview the layout at any size you'd like while you're editing.
The iframe UI works by using two "windows." One is the browser window you use to view the builder, also referred to as the parent window. The other is an iframe window used to render the page content for you to edit.
What's in the Parent Window? The parent window contains what is needed for the iframe to load and work correctly. This is also where the iframe renders the builder's UI once it has loaded. That includes the majority of the UI including the toolbar, menu, panels, and settings. Even though the parent window loads everything, the iframe controls most execution.
What's in the IFrame The iframe currently contains the entire page and builder. That could change in the future but