feature request: Aging of whitelist

[moschleg]

As noscript is used, the Per Site Permissions list gets very long. It would be useful for the least frequently used Per Site permissions to be pointed out. This way they could be removed making the list shorter so the plugin would work faster.

[barbaz]

Just to point out, this would be useless and misleading for people who run the browser in a sandbox that gets dumped on quit: the usage stats would have no bearing whatsoever on reality, with almost *every* entry being seen as never used.