remove explicit dependence on split-*-threshold

   Not all internal variables from window.el are loaded on emacs startup

git-svn-id: https://svn.r-project.org/ESS/trunk@5658 0bbaf3bd-34e0-0310-bf65-c717079852d4
update tracebug commentary

git-svn-id: https://svn.r-project.org/ESS/trunk@5659 0bbaf3bd-34e0-0310-bf65-c717079852d4