Allow user to Back Out of Add Properties Action During Upload

Summary:
During the drag and drop upload process there needs to be a means to back out of Adding Properties without cancelling the upload.

Steps To Reproduce:
1. Drag and drop some files into Nuxeo
2. As files are transferring click Add Properties button
3. No way to back out of this action without cancelling the entire upload process.

Results:
No way to back out of this action without cancelling the entire upload process.

Expected:
Either there should be a Back button since there is a Next button or else the Cancel button should function to cancel out of adding properties.