Create some example extensions that add list columns

[Nicklas Nordborg]

BASE 3.2 will add extension points for adding more columns to the table on list pages (​http://base.thep.lu.se/ticket/1674). We should include some examples in the example code.

1. A simple example that uses regular property paths. For example, the registration date of the owner of an item. This example should be added to the list pages of all ownable items.
2. One example that demonstrates the use of a custom formatter.... Have to think about this for a while...