Imports in DAM override media with same title/filename

In the new DAM version, there is no more support for "Import Sets", that used to put new media in distinct folders. All documents imported through the DAM view are now stored at the domain root, leading to an annoying consequence: we now can't have two pictures with the same filename, nor it seems two videos/sounds with the same title (according to the source code).

I guess it is not especially intended as in inherits the way the AbstractFileImporter works: depending on the importer, a new file will override any other one with the same filename/title, or fail if we don't have the Write rights to the existing file (1).

Given that the goal for Nuxeo DAM is to hold potentially huge amounts of media, the current behavior seems really wrong, so I suggest to disable the "overwrite by title/filename" feature in the DAM domain. It will probably require to update the AbstractFileImporter API + make the behavior customizable by DAM in some way, though I'm not sure what's the easiest way to do so.

(1) This can happen often and unexpectedly if, like in a project I'm working on, file rights are locally updated to make them private by default, i.e. Read/Write rights are removed from all users except the file creator.