[PoC] Add writing interfaces

A proof-of-concept to see how it would look like for the MediaBundle. We have had a discussion about separating setters and getters on the weekly irc-meeting in general for other bundles.

These related PRs need to be updated if the writing interfaces will be used:
- symfony-cmf/BlockBundle#72
- symfony-cmf/CreateBundle#60

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | na
| Fixed tickets | na
| License       | MIT
| Doc PR        | TODO, the whole MediaBundle needs to be documented