Fantastic tool very well done.
Just one small cosmetic suggestion, after the scan the list and addon title are not centered on screen.
See these image for better understand:
Before: http://oi64.tinypic.com/m82nep.jpg
After: http://oi67.tinypic.com/nfjpth.jpg

Great addon. Could you add an "exclude folder" option. Many session restorer addons and similar ones use the bookmarks as a backup and duplicates on those folders are very frequent and required so it would be nice to be able avoid scanning that. Thank you. Also it would be great if you could add a way to contact you like a mail or github in case someone wants to request a feature or report a bug.