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

include/wx/generic/dataview.h

@@ -229,6 +229,9 @@ public:     // utility functions not part of the API
     wxDataViewColumn *GetColumnAt(unsigned int pos) const;
 
 private:
+    virtual wxDataViewItem DoGetCurrentItem() const;
+    virtual void DoSetCurrentItem(const wxDataViewItem& item);
+
     wxDataViewColumnList      m_cols;
     wxDataViewModelNotifier  *m_notifier;
     wxDataViewMainWindow     *m_clientArea;