Retain views of "sibling" top controllers when using replaceTopController

Think navigation drawer selection that triggers a replacement of the visible top controller.

I'll use Google play music as an example.

1. Tap navigation drawer "music library" -> `MusicLibraryController` is created, view inflated, and attached.
2. Tap navigation drawer "podcasts" ->
`MusicLibraryController` is detached and it's view is destroyed
`PodcastsController` is created, view inflated, and attached.

When "Podcasts" is clicked, I want `MusicLibraryController` to be detached, but retain it's view without it being destroyed. Is this possible using a single `Router`?

Start with Router and the replaceTopController flow described in the issue, then trace how sibling controllers are detached and how their views are destroyed. Reproduce the navigation-drawer sequence with MusicLibraryController and PodcastsController. Done means a replaced top controller can be detached while retaining its view when using a single Router.

Written by the indexing model from the issue text.