display group's setSelectedObjects did not call setSelectionIndexes

```WODisplayGroup.setSelectedObjects(NSArray)``` directly sets variables holding selection information instead of calling ```WODisplayGroup.setSelectionIndexes(NSArray)``` as the Javadocs pretend. That means that when calling *setSelectedObjects* a display group's delegate will **not** be notified of the selection change.

This patch correctly calls *setSelectionIndexes* but for ERXBatchingDisplayGroups as *setSelectionIndexes* uses the private variable *_displayedObjects* which is redefined in ERXBatchingDisplayGroup.

If there are no objections this patch will be merged soon.