Only call mdls when there are new entries

Just checks to see if there are any new entries before needlessly
calling mdls.
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.
Merge pull request #9 from clintxs/safari-downloads

Add progress info for Safari downloads