Use item subtypes - step 2

[Nicklas Nordborg]

See also #335.

The first step defined subtypes and use them when creating new items. Before this step can be implemented, the pre-2.0 items must be updated with a subtype.

When that has been done we can use the subtype information when searching for items, which should make it easier to avoid pattern-matching problems like #324.

[Nicklas Nordborg]

(In [1492]) Fixes #334: Use item subtypes - step 2

All queries now use subtype filters which should be more stable when finding items.

[Nicklas Nordborg]

(In [1503]) References #334: Use item subtypes - step 2

Make sure that the subtype id has been loaded before it is used by a filter. Otherwise the query will not find any items.