Implementation of new tags API

Tags on files and blackboard artifacts are no longer implemented as blackboard artifacts. Three new tables are added to the case database schema. The SleuthkitCase class is updated to access the new tables and to update older case databases to the new schema (version 4), including migration of data. Data transfer objects (DTOs) are provided for tag names, content tags, and blackboard artifact tags. Basic support for future features invovling adding descriptions and colors to tag names and tagging content extents is in place.