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`....

- [ ] test coverage
- [ ] finish the code
- [ ] gather feedback for my changes
- [ ] submit changes to the documentation


| Q             | A
| ------------- | ---
| Bug fix?      | [yes|no]
| New feature?  | [yes|no]
| Tests pass?   | [yes|no|none]
| Fixed tickets | [yes|no|none]
| Doc           | [url to doc|none]