Don't show anything in the cells which should be empty in Cocoa wxDVC.

Implement a custom NSTableColumn-derived class to return nil for the cells
which shouldn't show anything at all because they are part of a container row.

This finally fixes the totally wrong display of the first page of the dataview
sample under OS X.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62493 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/osx/cocoa/dataview.h

@@ -498,6 +498,9 @@ public:
     wxDataFormat GetDnDDataFormat(wxDataObjectComposite* dataObjects);
     wxDataObjectComposite* GetDnDDataObjects(NSData* dataObject) const;
 
+    // Cocoa-specific helpers
+    id GetItemAtRow(int row) const;
+
 private:
     wxCocoaOutlineDataSource* m_DataSource;