Cleanup SearchIndexItem API

To support the work for the spec tag, JDK-6211738, it would be convenient to add a new field into the SearchItem. Related to this, it suggests the following:

1. Move SearchIndexItem and SearchIndexItems to the toolkit.util package
2. Refactor the code to create SearchIndexItem into a factory method, either as a static method in SearchIndexItem, or in a subtype-able factory class.
3. Possibly merge with IndexItem and/or IndexBuilder that already exist in the toolkit.util class.