Polish and papercut-fixes for the viewer

Since we want to release WebODF and ViewerJS very soon, this PR includes several bugfixes that make the viewer more polished.

Fixes:
- #474
- #573

Chromium has a nasty bug wherein `display: inline-block` is forgotten on the canvas element after a call to `fixContainerSize` (the CSS property is still there, but not visually applied). Setting it again on every `fixContainerSize` call solves the issue.

Also fixes another ugly behavior in which a slideshow page is shown aligned to the top of the screen, which is very annoying on a phone. It is now vertically centered.

Before:
![viewer-centered-false](https://cloud.githubusercontent.com/assets/591038/2983464/96bb8902-dc24-11e3-845a-6ee86c953277.png)

After:
![viewer-centered-true](https://cloud.githubusercontent.com/assets/591038/2983465/988ea534-dc24-11e3-8f41-5dfd09d41d53.png)