title: Warn user before closing Vaadin window with unsaved changes
body: <p>I'm having a Pop-Up Window implemented as a child window of the main window.
  I've an table with editable cells in it. (Basically it turns editable upon a double-click.)
  Please suggest how I can make this window throw a warning to the user when an attempt is made to close the window having unsaved changes. I know partly that ConfirmDialog can be used to address this. However, identifying the unsaved data is the challenge I face.
  I'm using Vaading 7.4.3</p>