[FEATURE] XML-Fileloader for snippet types

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes
- [x] submit changes to the documentation


__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | no => https://github.com/sulu-cmf/sulu/pull/257
| Fixed tickets | none
| BC Breaks     | none
| Doc           | https://github.com/sulu-cmf/docs/blob/master/detail-specification/600-global/DET-602-Snippets.md