This commit breaks the application as it introduces incompatible changes which
will be adopted bit by bit.
Since the filter functionality is subject to change it's removed from the BaseQuery.
The functions setOrderColumns() and getOrderColumns() are deprecated because they're
replaced by functions in the Sortable interface.
Further the Sortable interface now defines the sort constants ASC and DESC and are
thus removed from the BaseQuery. In addition the sort constants are no longer integers
but strings.
The distinct() and isDistinct() functions are removed because they're database specific.
They have been introduced with changes in the PivotTable implementation which should
actually not call distinct blindly anyway.
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