[WIP DNR] Allow wrapping screens to guarantee a described view controller

[kyleve]

This PR introduces a wrapper screen that transitions its content when the underlying VC type changes; which was lost as a "standard" thing when we removed DescribedViewController usage from our apps – since it effectively doubled the VC hierarchy depth. Some folks still want this behavior, so giving them a more concrete way to achieve it.

• Add AnyContentScreen, which is a screen-based wrapper around DescribedViewController. If the containing screen's VC changes, it'll transition per the added transition type...
• Speaking of which, added the concept of an animated transition to DescribedViewController when it replaces its content. This in general probably always should have been a thing; since without an animation, things look pretty sudden / abrupt.

Checklist

• [x] Unit Tests
• [x] UI Tests