LMS-749 minor: extract inner classes, add javadoc

...lient/application/model/DataSetSearchFieldComboModel.java

...lient/web/client/application/ui/data/CriterionWidget.java

...plication/ui/data/DataSetSearchFieldsSelectionWidget.java

...neric/server/dataaccess/db/search/LuceneQueryBuilder.java

...nbis/generic/shared/basic/dto/DataSetSearchCriterion.java

.../openbis/generic/shared/basic/dto/DataSetSearchField.java

...nbis/generic/shared/basic/dto/DataSetSearchFieldKind.java

...eric/client/web/client/application/DataSetSearchTest.java

...nt/web/client/application/ui/data/FillSearchCriteria.java

.../generic/server/dataaccess/db/HibernateSearchDAOTest.java