cleaning up label.py a little bit, consolidating trait change handlers
cleaning up label.py a little bit, consolidating trait change handlers

tommy-u/chaco,burnpanck/chaco,tommy-u/chaco,ContinuumIO/chaco,burnpanck/chaco,burnpanck/chaco,ContinuumIO/chaco,ContinuumIO/chaco,tommy-u/chaco,ContinuumIO/chaco