title: How to flag an invalid entry with CellRendererText
body: <p>I'm writing an application with tables full of user-editable data. Some of the table columns contain numbers. I want the user to be able to type a new number in to a field in the table, but if the user types "foo" or "1.2.3" I need to be able to signal to the user that this has not been accepted.</p>
  
  <p>When using a normal Entry widget I can set the state to Invalid if the user enters something that is not a number. However for the tables I'm using TreeView with TreeViewColumns. The user is editing text displayed with CellRendererText. I know that I can get the actual text entry widget while the user is entering text, but as far as I can tell when the user presses "Enter" that widget goes away before the CellRendererText sends the TextEdited signal.</p>
  
  <p>What is the right way to signal to the user that they have typed in the wrong thing in this circumstance? I'd like to do something like outlining the offending field in red.</p>