Can't select which update site provides a file

See http://fiji.sc/bugzilla/show_bug.cgi?id=1024

The [Bio-Formats](http://sites.imagej.net/Bio-Formats/) and [IMCF](http://sites.imagej.net/UniBas-IMCF/) update sites both provided a `Stitching.jar` implementation. As far as I can tell, the `Bio-Formats` version is always overwritten by the `IMCF` version, regardless of the order the two sites are enabled.

It would be nice if we could use the `update site` column to choose which update site to receive a given object from.

Start by reading the linked Bugzilla report and tracing the update-site column in the ImageJ Swing UI. Determine where duplicate files from enabled sites are resolved; done means the update-site column lets a user choose which provider supplies a given file, including the duplicate Stitching.jar case.

Written by the indexing model from the issue text.