Thumbnails calculated on version which already have one

Problem: Every time a version is created, a thumbnail is computed, while this is useless when the live documents already has a thumbnail.

Check/Reproduce:

• Nuxeo 10.10 out of the box with nuxeo-web-ui only
• Set a break point in org.nuxeo.ecm.platform.thumbnail.listener.CheckBlobUpdateListener
  • See this permalink to GitHub
• Upload a file => breakpoint reached (normal)
• In the UI, create a version => breakpoint reached (not normal)

Discovered because I update my demos by setting dublincore (changing modification dates and users => as user is not the same, versioning policy creates a version => my computer was full of thumbnail calculation).

Important: I believe it will be worth also checking videos are not transcoded a second time too.