Versioning

We can easily keep old versions of PloneExFiles. Here is what needs
to be done:

- Show the list of old versions
  - file, version, annotation [download link]
- When uploading a new file:
  - Checkbox: "keep old version?" (yes by default)
  - String: "Version" (Propose a default, allow user to change it)
  - Text: "Changes" (Empty by default) 
    - Allows user to annotate the current version that he uploads
  - We reuse what was done in PloneArticle to have a list of files
    - Before overwriting the old file, it is added in front of the
      list like it is done in PloneArticle
    - We store the new file just like it used to be in PloneExFile,
      thus remaining backwards compatible
