Add dynamic elements

[wagoodman]

Enhances the frame object to be able to:

• prune elements that are claimed to be dead (via a new TerminalElement interface)
• prune elements that are claimed to be dead, but should show up as a trail of text behind the UI block (via a new ImprintableElement interface)
• show/hide elements based on the model state (via a new VisibleElement interface)

This allows for introducing items to the UI that can self-destroy, collaborating with the owning model on the details.