For Picture documents: Make images used for Thumbnail and display configurable

As of today, the images displayed for the Thumbnail and the view in "Full layout" are hardcoded:

• Thumbnail uses the "Small" picture:views. If not found, looks at the "Thumbnail" picture:views. If fails, default icon. (This is the algorithm of ThumbnailPictureFactory)
• Display in Full layout uses "FullHD" picture:views

It would be very cool to be able to define which picture:views to use for Thumbnail and which to use for display.

Notice:

• The ThumbnailPictureFactory can be overridden to return any picture:views for the thumbnail.
• Overriding the URLs used in JSF/WebUI for the main display may be a bit more tricky. JSF hard codes the "FullHD" value, WebUI - as of "today", 8.10 and 9.1-SNAPSHOT-2016-12-10 uses the #4 element of picture:views (assuming it's the "FullHD" view)

But I am talking about configuring easily, no need to write a Java class or change default widgets.