Add a new "velocity" value for the "contenttype" TextArea meta-property to display velocity code in an XObject's property

We have many cases when we store velocity content in XObjects. They are usually stored in TextArea fields but have no displayers so they need to be parsed by the calling application.

There already exists a "velocitycode" value for the "contenttype" meta-property of a TextArea, but it is only used if the XObject's document is in 1.0 syntax and to display it with XWiki.parseContent(), which is not nice.

This new value should be handled with the VelocityManager/Engine and nothing more.