Allow custom sorting syntax with "<field> => <sort>".

Until now, no custom sorting syntax (= custom SQL) could be used.
Internally DCG only work with the field name as reference, but the
sorting is done by the data provider. Now it is possible to define
custom sortings by using "inColumn => FIELD(inColumn, 'header', 'main',
'footer')" for example.

This behavior must be supported by the data provider!