Fix selection event generation in wxMac wxListBox

Prevents deselecting the selected item in single-selection listbox.

Also generate correct events in the multi-selection case by reusing the
existing wxListBoxBase::CalcAndSendEvent() method.

Closes #15603.

include/wx/osx/listbox.h

@@ -125,6 +125,12 @@ public:
     bool MacGetBlockEvents() const { return m_blockEvents; }
 
     virtual void HandleLineEvent( unsigned int n, bool doubleClick );
+
+    // These are called by wxNSTableView
+    using wxListBoxBase::DoChangeSingleSelection;
+    using wxListBoxBase::CalcAndSendEvent;
+    int GetOldSelection() const { return m_oldSelections.empty() ? wxNOT_FOUND : m_oldSelections[0]; }
+
 protected:
     // callback for derived classes which may have to insert additional data
     // at a certain line - which cannot be predetermined for sorted list data