Make it possible to force users to input metadata after drag&drop

At the moment users can create documents using drag and drop and completely skip the metadata input. That may be ok for some use cases but there should be an easy way for application builders to force metadata input if the use case requires so.

Possible solution:

• use the CREATE_POPUP_ITEMS/CREATE_POPUP_PAGES slots for nuxeo-document-import instead of hardcoding it in nuxeo-document-create-popup
• add a param to nuxeo-document-import to disable the create button in the "upload" stage

today the only way to do this is to customize nuxeo-document-import which is very bad for maintenance and inject the custom element in the CREATE_POPUP_PAGES slot and give it the same id/name as the one hardcoded in nuxeo-document-create-popup (that's bad because the DOM now contains the original nuxeo-document-import element that is not used at all)