Add a drop zone widget type

There are several places in Nuxeo where a drop zone is defined.

Using a widget type to do that would be useful.

This is mainly a div with class "dropzone".

Required properties:

• dropContext: the drop context (used to retrieve automation chains to call on drop)

Additional optional properties:

• styleClass: additional style class to set on the div after "dropzone"
• cannotDrop: an EL expression, if it resolved true, the drop zone will not be activated
• isEmpty: boolean expressing whether the drop zone should be empty. If it resolves to false, subwidgets are displayed inside the drop zone, else the emptyMessage property is displayed.
• emptyMessage: message to display inside an empty drop zone area
• cannotDropMessage: message to display when user cannot drop (only displayed when drop zone is empty)