QAspectEngine owns the root of the scene

The scene root is now also enforced to be a QEntity as that is what we
were checking for in the backend anyway as it is impossible to create
a plain QNode.

This is another step towards solving the shutdown crashing bug.

Change-Id: I8508afa80ec9e99954ab867be1ed28bc35405e79
Reviewed-by: Paul Lemire <paul.lemire@kdab.com>
Cleanup the scene before quitting the backend

This gets us closer to a crash-free shutdown. It is consistently
crashing inside of the cleanup of the font cache's thread local storage
now.

Change-Id: Idbed73aa9ccedb32632cd6f19183827052697975
Reviewed-by: Paul Lemire <paul.lemire@kdab.com>