Use G_SOURCE_CONTINUE/REMOVE

Now that GLib provides these macros, we should use them
to make the code more readable.

gtk/gtkprintoperation-unix.c

@@ -1068,7 +1068,7 @@ find_printer_idle (gpointer data)
   
   printer_finder_free (finder);
 
-  return FALSE;
+  return G_SOURCE_REMOVE;
 }
 
 static void