Add the possibility to configure a content-view as a drag and drop zone.

According to http://doc.nuxeo.com/x/BAJu, it is necessary to use an html element to make a zone drag and dropable. To use it with content-views and Studio, it means redeclaring a full drag and drop widget and include the content-view in it.
As most of the time, drag and drop will be on content-views. it would be nice to be able to set up the drag and drop properties directly on the content-view (make easier to use with Studio for instance).