WebSearch: get_index_id_from_field() look in index

• So far, get_index_id_from_field() looked only in logical fields in order to find out the index ID corresponding to some field. This necessitated to have at least some field defined for an index. After this patch, we shall find index ID also via index name, even when there is no logical field of that name associated to it. (Example: index foo' consisting of two logical fields author' and title' is now usable in queries like foo:bar'.)