Fileset display

In the web thumbnails center panel, this PR subtly indicates if you've selected some of the images within a fileset, by outlining additional thumbnails in that fileset.

This might help users to understand that some actions will be affected by filesets, although this needs some thought, discussion etc.

![Screen shot 2013-04-24 at 16 18 01](https://f.cloud.github.com/assets/900055/420587/351acef4-acf2-11e2-8903-dbe9b63c44f7.png)