Undo not cleared when opening new file

[GoogleCodeExporter]

Open a file, makes some changes, open another file, you can still undo which 
will bring you back to the back to the previous file's layout, and you cannot 
redo back to the file you actually have open.  Saving will then overwrite your 
current file with the previous file's layout.

Original issue reported on code.google.com by ggbb...@gmail.com on 5 Sep 2013 at 4:58