Make it possible to update an object in the object registry.
Allow a default value for utils.get_object.
Add utils.delete_object()
Use object registry for last focused tab.
Move object registry to its own file.
Make pylint shut up with _UNSET object.
Also use objreg API in app
Detach object registry from application.
Detach meta object registry from application.
Use object registry for url_stack.