Switch from Venusian to tangled's equivalent system

- Application.scan() renamed to Application.load_config()
- tangled.app.scan setting was renamed to tangled.app.load_config
- @config and @subscriber decorators were updated and simplified
- @config tests updated and simplified

Motivation: Venusian's implementation is really complex. 1.0 introduced
a bug that hides method decorations when the class is also decorated.