Render container and component outlines inside the iframe

After the site has loaded, 'Edit' mode should show blue component outlines and black container outlines (only for empty containers). The outlines should be rendered inside the iframe, so the iframe is always as high as the app itself. The overlay moves along with the site when the site is scrolled.

Depending on the OS, the vertical scrollbar will take up some horizontal space in the iframe. The iframe contents will then be a little less wide than the width of the selected device (e.g. not 720px for tablet, but 720px - 15x scrollbar = 705px). That is fine. The Material Resizer does the same.

The scrollbar of the iframe is rendered by the iframe itself, so 'Tablet' and 'Phone' mode will have a scrollbar right next to the iframe instead of at the right side of the app. The scrollbar will scale down with the left side-panel is opened.