Add a new macro to provide a way to focus on a modeless dialog with keyboard

This was motivated by repeated experiences of "I need the mouse"
frastration:
  1. Open Search&Replace dialog.
  2. Switch to another application. (Alt+Tab)
  3. Switch back to jEdit. (Alt+Tab again)
Then the view got focus and unable to focus back to Search&Replace
without the mouse. This happend with "File changed" dialog, or some
others.