#11515: wxDataViewSortedListModel get/setters

Patch:
Get/setters to access private data members.


Notes:
- The reason for this patch is simply that I have several projects that cannot compile without it (sf.net/projects/dbase, sf.net/projects/regattalog, cpcloader.trak.dk, wxcommander.com)
- wxDataViewSortedListModel does not exist in 2.9 so these projects comes with their own wxDataViewSortedListModel implementation for 2.9, but naturally I'll prefer using the official wxDataViewSortedListModel implementation when building with 2.8 - which requires this patch to be applied to work.
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