Fix display of items without attributes in Cocoa wxDVC.

Fix display of items without attributes in Cocoa wxDVC.

The attribute used for the last item was reused for the next item in the same
column unless it was overridden in the attribute of this item, fix this by
remembering the original attribute and using it if no attributes are
explicitly specified.

Also change the sample to show the items without attributes in a column with
attributes and make the label correspond to the attribute of the item.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62390 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775