title: Change label on an mxgraph cell/vertex
body: <p>I've created an interface, including mxgraph (javascript), that allows a user to click on a cell and edit the cell's label in a regular form text input that's separate from the graph.  I'd like to change the cell's label after the user saves the form (ajax submit) without having to reload the graph.</p>
  
  <p>I really don't need to do anything fancy with a cell/vertext label, but I do need to be able to change it's text.  I don't need HTML labels if that's avoidable.  And, I haven't found something like a "setLabel" method in the API.</p>
  
  <p>Sounds like a simply thing.  What am I missing?</p>
  
  <p>Thanks</p>