title: NSTableView - how to change string just before editing
body: <p>I have a table of file names. I want the user to be able to change values but i want to allow changing only the name without its extension. </p>
  
  <p>So, if there is a cell with the value filename.zip and the user start editing that cell i want him to be able to edit only the filename part without the .zip extension.</p>
  
  <p>How can i do that?</p>