Computed field values should be available in REST representations

Currently, a computed field is computed when a page containing an object with such a field is rendered in view mode, or in livetables. It can be useful to retrieve such a value not only visually in HTML but also via XML or JSON rendering via a REST call. This issue is about proposing that the computed field values get computed in various cases that are currently not covered, in particular when returning a REST representation of an object.