User should be able to control if assets with the same name are overriden

When you drop an asset that already exists through Nuxeo DM the new asset replaces the first one and a minor version is created.
When you drop an asset that already exists through Nuxeo DAM the new asset also replaces the first one, except that:

• you don't really know where you drop your asset(s)
• so you don't know which item already exists
• you're not notified if one of your new asset have overriden an existing asset
• you're not able to control version of your assets in DAM

The related NXP-12136 issue have to be resolved first.