Add wxDataViewCtrl::{Set,Get}CurrentItem().

Current item is the same as the selected item in single selection mode but in
multiple selection mode there was no way to neither get this item nor change
it before so add the new functions to allow doing this now.

The new methods are implemented for the generic, GTK and OS X/Cocoa versions
but only stubs are provided for OS X/Carbon.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65228 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/changes.txt

@@ -411,6 +411,7 @@ MSW:
 All (GUI):
 
 - wxAUI: support auto-orientable toolbars (wsu).
+- Added wxDataViewCtrl::Set/GetCurrentItem().
 - wxHTML: render in RTL order inside RTL window (Richard Bullington-McGuire).
 - wxRibbon: added EVT_RIBBONGALLERY_CLICKED event (John Roberts).
 - Add support for CP-866 encoding to wxEncodingConverter (madnut).