Upload a folder (and its arborescence) by drag & drop

A user need to be able to Drag and Drop a folder as he is doing with files.
Moreover, if the folder contain sub-folders, they need to be created too with their own content.

The drag&drop area would take place at 2 different place.

• On a folder (tree) => This will add the dragged folder (and its belongings) in the drop folder. It will also create the folder tree respecting the one included in the dropped folder.
• In a folder (browser) => This will add the dragged folder (and its belongings) in the opened folder. It will also create the folder tree respecting the one included in the dropped folder.

Since this operation might take ages and consume a lot we need to add a warning to ask to the user if he is really sure that he "wants to upload X files and folders at once".