Add sort-by to filter options

[mr-ransel]

Specifically, when browsing the list of known/previously detected trackers, it would be helpful to be able to sort by number of times seen vs recency, and the same for the scanning page, possibly with the ability to sort by signal strength as well.

The value of this is that normally as a user if I'm trying to label trackers I recognize or identify trackers that haven't yet risen to the "alertable" stage, it's likely that the most interesting items would be the ones most frequently seen, or in realtime those with the highest signal strength, but it would depend on user preference.