a plugin that hooks on EVENT_FILTER_COLUMNS has to return an array of classes that inherit from MantisColumn.

After thinking on the scenario in this request:
https://www.mantisbt.org/bugs/view.php?id=13180#c51538
I feel using only the class name not versatile enough. It wont allow programatically manage this functionality, because there can only be one class defined for each class->column we would want to add to the system.

If EVENT_FILTER_COLUMNS can accept objects (instances of class that extends MantisColumn), plugins can provide objects with different column names and behaviours at runtime.