Updated overlay.js to support nested overlays -- in other words.
1. On Page A, click Link A. Link A opens Overlay A.
2. In Overlay A, click Link B. Link B opens Overlay B over Overlay A.
3. Close Overlay B to return to Overlay A.
4. Close Overlay A to return to Page A.
In theory, this should work for an infinite number of overlays that open other overlays (or up to the max size of an array before Javascript crashes).
A two-level Overlay can be seen in the Area Manager. When adding/editing an Area, the Edit overlay appears. In the Edit overlay, clicking the [Add New Prefix] option opens the Edit Prefix overlay, which will update the Area Edit overlay on submission.