The grab code was trickier with current API (which is why I left it as
is), but I took another look and it turns out that code has never
actually been used...

So I removed _grab_lock() and _release_lock(), and replaced STOCK
items with text.
I've added these changes to the same branch (should now compile
without warnings):
  git://github.com/jku/sato-screenshot gtk3
  https://github.com/jku/sato-screenshot/tree/gtk3