title: ExtJS extending NumberField to CurrencyField
body: <p>There is a very nice Answer how to enforce decimal precision in <code>NumberField</code>: <a href="https://stackoverflow.com/questions/1287584/how-do-i-force-the-display-of-a-decimal-in-an-extjs-numberfield-to-a-certain-prec">How do I force the display of a decimal in an ExtJS NumberField to a certain precision?</a>.</p>
  
  <p>But it only works half way. I would like to do 1 step more and implement a thousands separator and dollar sign (US Money). I tried to extend <code>baseChars</code> to '1234567890$,' but it did not work.</p>
  
  <p>Is someone have this already solved or know how to do it?</p>