External column handler

[szabogabriel]

Add a configuration option to define classes from the classpath to be used as column data sources.

The usage of such classes must be configured via configuration by using the callback:/[domain]/[category]/[level_n] key and the absolute class name as the configuration value.

The class itself must implement a common interface, which will be used to retrieve data for the given column.

Caching and other performance tuning must be performed on the external library site.

[szabogabriel]

Fixed