[FEATURE] Added extension alias and magic getter in Extension to shorten data access

Added extension alias and magic getter in Extension to shorten data access

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | https://github.com/sulu-cmf/SuluContentBundle/issues/159
| BC Breaks     | no
| Doc           | none