Alert script authors when Greasemonkey is sniffing for grants

[arantius]

If we see a script modified on-disk, assume the user is a script author. If so and there is no @grant metadata (so it gets sniffed), throw some sort of warning at the user.

Ideally, with https://developer.mozilla.org/en/Using_popup_notifications if we also make minversion==4.0 in this release. Where actions would be to open detailed (wiki) documentation, and dismiss future warnings like this.