FetchImageOperations can now report the progress of their downloads

Also made the interface show the download progression by adjusting the shading of the file name info box.
Download progress indicator reflects previous progress

If image cell went offscree then back onscreen it wouldn't reflect previous download progress until it receive a new chunk. Now it remembers previous progress.
Merged thumb download progress indication on to latest
Merge pull request #67 from montehurd/downloadProgress

Download progress indicator for thumbnails