Instead set this on inidividual categories, as that is where it actually
gets used. This makes it easier for SqlCompletionCategory to reuse a
prepared query (as it gets the filter field names in its constructor).
The history completion query is extended to pick only the most recent item for
a given url.
The tests in test_models now check for ordering of elements.
Now that sql is only used for history (not quickmarks/bookmarks) a number of
functions are no longer needed. In addition, primary key support was removed as
we actually need to support multiple entries for the same url with different
access times. The completion model will have to handle this by selecting
something like (url, title, max(atime)).
This also fixes up a number of tests that were broken with the last few
sql-related commits.
