thesaurus and thesaurus keyword indexing

[simonp]

Now that keywords (aka terms or concepts) chosen from a thesaurus in GN can have their id/code as part of the metadata record (eg. in a gmx:Anchor), the id of the keyword can be in the index with the thesaurus as the field name.

As an example of how useful this will be: it will allow a thesaurus shown in an extjs widget in the search interface to be (optionally) filtered to remove any keywords that aren't in the index. This is better than just loading the widget with the keywords from the index because a widget loaded with the thesaurus also has the descriptions of the keywords to guide the user in choosing which keyword to search on.