Add a "Sidebar" component

[Windvis]

Loket (and derived apps) have been using a custom sidebar implementation which is used to display the details of some records. Other projects also want to use a similar setup, so it makes sense to add it to Appuniversum now.

The actual requirements are still to be determined, and a design might be needed if the Loket version needs changes. AFAIK Webuniversum doesn't have something like this.