Scene nesting

[tobiasBora]

Description of proposed feature

If would be fairly cool to be able to nest scenes. For instance, I could create a 3D animation, and put it next to a 2D animation. This may also be useful to implement a rudimentary cropping (related #2625).

How can the new feature be used?

Here is the image shamefully stolen from BioAbner, taken from its question on discord (but one could imagine 3D animations instead of 2D):