Rewrite: Named Snapshot History Management Class

This is a rewrite of the named history database wrapper that was called `TagList` before. Now this responsibility is fulfilled by the `History` class. It fully hides the SQLite details from the using code. Hence, all database queries and the SQLite connection is handled by `HistoryDatabase`. Note, that this Pull Request does not contain necessary API changes in the user code and hence will break the build when merged alone.

Additionally there is a unit test set for the `History` class.
Yeah, true. The `LIMIT 1` doesn't really do anything, but also doesn't hurt. ;-)