Add foreground, background layers

[bollwyvl]

Elevator Pitch

Enable placing whole cells, or just outputs, in the background or foreground of the whole deck, or just a slide.

Motivation

Being able to use (potentially dynamic) content would be a powerful technique for both theming as well as combining out-of-band content.

Design Ideas

• offer these as jss-compatible recipes which can be added to cells
  • might "just work" with Support per-cell styling jupyterlab-fonts#47
• use custom metadata
  • probably better fine-grained control