* Title: Content version diff

*Documentation:
Added possibility to view differences between versions of content objects.
The diff is run on the datatype level. The default diff action is to show
the old and new version at the same time. For text and xml-text a diff of
the text content itself will be shown. The feature is accessible from the
popup-menu in the administration interface.

Though, at this moment there exists some bugs in diff of text, and
xml-text will for the time being only display old and new version.