Add JDBC import support

This adds the possibility to import data through JDBC (on both levels: API and GUI).

The API offers index and name based addressing, i.e. you can refer to a single column by its database name or by its index within the table. Regardless of the used addressing scheme you can always assign an alias name for each column, which effectively comes down to renaming the column on-the-fly in the output. The notion of name based addressing made it necessary to refactor the class describing single columns. There is now a dedicated class for each type of configuration, e.g. CSVColumn for CSV files. Refer to `Example2c`, `Example2d`, `Example2e` for details on how to use the API. For now JDBC drivers for MySQL, PostgreSQL and SQLite are included.

The GUI distinguishes between local (SQLite) and remote (MySQL, PostgreSQL) database types and offers different controls in each case.