Fix [#37039] Resize lists by new grab handles puts file into unsaved state.

Simply do not enable undo for LISTBOX buttons (the same as ROUNDBOX & co). Many thanks to Campbell who did all the work here!

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@60683 954f8c5b-7b00-dc11-b283-0030488c597c