[WIP] Started refactoring for Snippets

- Make Structure a super type
- "Page" will take role of current Structure
- Snippet will be subclass of Structure
@wachterjohannes @danrot @chirimoya I introduced this method to the content mapper, which I think should deprecate the `save` method. Basically to reuse logic we need to pass the content type (`Page`, `Snippet`) here -- but I will be damned if I add argument number 12 :)