title: style for GXT menu button
body: <p>I see GXT Menu can only be set on CellButtonBase subclasses' instances through setMenu() method. 
  I'd like to show an image instead of a button and show a menu when user clicks that image. unfortunately, Image is not a subclass of CellButtonBase and thus I can't attach a GXT Menu to it.
  so how can I make TextButton (which seems to be my only choice here) look like an image if I have to use it?
  There's no documentation or examples on this subject. I asked on Sencha GXT forum support, but got no response. </p>