Do not access private Renderer._actors property

[user27182]

The BackgroundPlotter accesses a private variable of 3rd party imported class, i.e. Renderer._actors. But there is no need to access a private property here since Renderer.actors simply returns Renderer._actors already anyway.

See Renderer.actors source:
https://github.com/pyvista/pyvista/blob/5be64528b73a970df364b4e2dd7ebfa0b62e9b64/pyvista/plotting/renderer.py#L818-L821

The underlying implementation for Renderer._actors is being modified by pyvista/pyvista#7184 and the access to a private variable here by pyvistaqt is causing integration tests to fail unexpectedly.

EDIT: This was addressed upstream by implementing an _actors.items() method, so this change is not strictly needed by pyvista/pyvista#7184, but I still think access to private vars should be avoided here