[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

- <strike> Sub-class the template reader. </strike> (handled in separate PR)
- <strike> Enforce the `title` property on Snippet templates.</strike>
- [x] Webspace should not be required by the API
- [ ] Either refactor the ContentMapper test or not. but in anycase, add Snippet tests^
- [x] Handle "not found" snippets when retrieving multiple snippets by ID (do not 500) - (fixed in SnippetBundle)

^ The snippet mapping is already well tested by the SnippetBundle, but we should add tests here too.
Also you can actually see what you are doing:

````
        $request = ContentMapperRequest::create()
            ->setType('page')
            ->setTemplateKey($page['template'])
            ->setWebspaceKey($webspaceKey)
            ->setUserId($this->userId)
            ->setState($page['published'] ? StructureInterface::STATE_PUBLISHED : StructureInterface::STATE_TEST)
            ->setIsShadow($page['shadow'] ? true : false)
            ->setShadowBaseLanguage($page['shadow'])
            ->setLocale($page['locale'])
            ->setData($page['data']);
````