Make it easier to change the automation chain called by element 'nuxeo-document-import'

Element nuxeo-document-import calls directly automation operation FileManager.Import when a file is dragged and dropped. As the element has no attribute to define another automation chain to be called and it is not recommended to override automation operation FileManager.Import (as it is a basic operation in building chains in Studio Modeler), an easy fix would be:

• in the Nuxeo Platform, define a new automation chain named i.e. FileManager.ImportInWebUI that calls operation FileManager.Import
• call chain FileManager.ImportInWebUI in element nuxeo-document-import

This way, it is easier to change the import behaviour by overriding chain FileManager.ImportInWebUI.

Another option is to add an attribute to the element to provide the name of the automation chain to call.