#125494 Part 2: Preview Changes button

This is a small part of the monolithic patch http://drupal.org/node/125494
Hopefully by breaking them up we can move them through faster.

This part deals with:
3. Preview changes button can be set per node type (admin > content types > edit)

I'm not really sure if this is necessary, but since someone requested it and the addition was minimal, I added it so we can see how it looks and behaves.

4. Preview changes button is available to all, you don't need 'view revisions' permission.

It's convenient for everyone to be able to preview the changes, even if the revisions itself cannot be accessed by that user.