show comments for presentations

[gregwebs]

I don't know how to cleanly show comments for a presentation. This is particularly true when in "full screen mode". I think up in the title bar area there could be something to click to see comments, much like the "Open comment history" in Google itself. However, I think this means the comments would never be viewed. I want to show the comments for a slide when a user views a slide. This might be possible with the iframe if the user has the extension installed: perhaps the extension can communicate the slide number back to the gdocwiki window.

Assuming we can do the above I think it is possible to add comments to an implementation of a sidebar slide preview described in #142.