Listener for setting current portal

Added a listener for setting the current portal
@chirimoya The listener is called after the RouteProvider, so we have to call the `findByUrl`-method twice (RouteProvider + KernelListener). I don't really want to set the current Portal in the `RouteProvider`....

- [x] test coverage
- [x] finish the code
- [x] gather feedback for my changes
- [x] submit changes to the documentation


| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | none
| Fixed tickets | no
| Doc           | https://github.com/sulu-cmf/docs/blob/master/detail-specification/000-foundation/DET-004-Routing.md