Updated undo patch

[Amos Brocco <amos brocco gmail com>]

Hello,
I've updated the undo-redo patch to clean a little bit the mechanism (based on
the work done by Cosimo in the undo-manager branch). It applies to current git
master.

The main differences are:
 - better code organization
 - separate undo data structures
 - fixes to let the patch apply to git master (mainly changes from
FMDirectoryView to NautilusView)

please find the patch at:

https://bugzilla.gnome.org/attachment.cgi?id=179634

cheers

A.