Issue #1354386 by thehong, florisla: full compatibility with insert.module

Not functional yet.

See http://drupal.org/node/1354386

Code copied from thehong's sandbox module multi_insert at
http://drupal.org/sandbox/thehong/1496560

Similar the issue 1533648, which adds the same support for multiupload_filefield_widget.
http://drupal.org/node/1533648