Don't lose selection in wxDataViewCtrl::Select() under Mac.

Calling Select() for an item should not deselect the other selected items in
multiple selection mode. This is consistent with the GTK and generic versions
behaviour and also just makes more sense but also document that this is indeed
the desired behaviour just in case.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65226 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/osx/cocoa/dataview.mm

@@ -2096,7 +2096,7 @@ void wxCocoaDataViewControl::Select(const wxDataViewItem& item)
 {
     if (item.IsOk())
         [m_OutlineView selectRowIndexes:[NSIndexSet indexSetWithIndex:[m_OutlineView rowForItem:[m_DataSource getDataViewItemFromBuffer:item]]]
-            byExtendingSelection:NO];
+            byExtendingSelection:GetDataViewCtrl()->HasFlag(wxDV_MULTIPLE) ? YES : NO];
 }
 
 void wxCocoaDataViewControl::SelectAll()