Implement a preview tab

Goal:
- A preview tab or pill for the documents tab, which renders previews
- The implementation is generic, means it's possible to create previews for every file type you can implement it :smile: 
- IWorkspacePreview adapter, which handles the representation of a preview.
  - It's a named adapter, the name is the extension of the file. Extension lookup through the mimetypes_registry
  - Handles multiple extensions, like jpeg/jpg/pjpeg (just register one adapter)
  - Default adapter for all types (currently shows a "don't panic" image :smile: )
- Implement image preview adapter based on plone.app.imageing for png, gif, jpeg
- Implement ftw.file preview for images, because we store images in files 0_o (historical). 
- Implement infinite scroll.

cc @jone 

Any suggestions?

I guess if this is a success we can implement this feature in his own package