[WIP] Refactoring for Snippets

- Make Structure a super type
- "Page" will take role of current Structure
- Snippet will be subclass of Structure

## Depends

- [x] https://github.com/sulu-cmf/SuluTestBundle/pull/25 
- [ ] https://github.com/sulu-cmf/SuluTestBundle/pull/26

## Todo

- [ ] Sub-class the template reader.
- [ ] Enforce the `title` property on Snippet templates.
- [ ] Either refactor the ContentMapper test or not. but in anycase, add Snippet tests^

^ The snippet mapping is already well tested by the SnippetBundle, but we should add tests here too.
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