Set event string field for wxOwnerDrawnComboBox events

Call wxCommandEvent::SetString() for consistency with the native
wxComboBox.

src/generic/odcombo.cpp

@@ -238,6 +238,8 @@ void wxVListBoxComboPopup::SendComboBoxEvent( int selection )
     evt.SetEventObject(m_combo);
 
     evt.SetInt(selection);
+    if ( selection != wxNOT_FOUND )
+        evt.SetString(m_strings[selection]);
 
     // Set client data, if any
     if ( selection >= 0 && (int)m_clientDatas.size() > selection )