[permissions] A "dismissed" state (#4)

Having read over the spec again, and in light of [Mozilla Bug 675533](https://bugzilla.mozilla.org/show_bug.cgi?id=675533), think we might need a `dismissed` state. Dismissed differs from denied, in that the permission prompt was discarded and no decision was made. 

I know it overlaps with `prompt`, but prompt really means "undecided" or not yet asked. But that's a different bikeshed.  

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