Invoking Find from Tracker window should default to "active" filetype in search dialog

[grexe]

Use case: When I have a mail/people/artwork folder open, I would find it handy - almost expect - to have Find default to the "active" filetype instead of always falling back to the generic default.

The "active" filetype should be deduced via heuristic, which is easier in list/details mode, where Tracker could check for displayed attributes and infer the most useful filetype from there.

This would help to present users with a more detailed and useful search result and fulfill one of the core rules of search.