'Transparent' screens

[nhaarman]

It may be possible to show a Scene while the previous scene is still visible, such as with dialogs.
How would this be handled?

[nhaarman]

The goal is to have something like this:

The tricky part here is that there are two scenes active, and both need to have their Containers attached.

In theory, this is a single Scene that contains two sub Scenes, so the single Scene could delegate its lifecycle and attach/detach methods to the sub Scenes.
The difference with #10 however is that when navigating from the first Scene to the second Scene, we ideally don't want the first Scene to go through the onStop- onStart lifecycle.
Detaching and attaching the Container shouldn't be an issue though.

[nhaarman]

An experimental API is presented in #121, docs can be found at https://nhaarman.github.io/Acorn/wiki/experimental/concurrentpairnavigator/