Add progress info for Safari downloads

Adds a progress icon and a subtitle with download progress to Safari downloads (.download files). A .download file is really just a folder containing a plist and the partial download, so it's easy to extract the data from it. I would've liked to get it working with Chrome, but it seems like it just stores everything in a single binary file, so I'm not sure how feasible it would be to extract that data.

![Screen Shot 2013-04-01 at 8 17 55 PM](https://f.cloud.github.com/assets/375202/326426/ea1f5f64-9b35-11e2-9ace-b6f26b39e953.png)

Also made a change to only call mdls if new entries were added.