Fixing IE6 CellTree bugs.  Child nodes disappear after the open animation completes, and images are not located correctly.

Review at http://gwt-code-reviews.appspot.com/997801

Review by: rice@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9059 8db76d5a-ed1c-0410-87a9-c151d255dfc7