Add general API to insert external images/sprite in draw order

[badlogic]

Some runtimes provide a way to interlace images/sprites external to the skeleton with the skeleton's attachment and draw order. We need a generalized API to be implemented by runtimes to do this. It may not be part of the core runtime API, but should be similar enough between concrete runtime implementations.

[badlogic]

This is not possible in an engine/framework independent way. Most runtimes now have examples on how to achieve this, except SFML and SDL.