Fixes pseudo-memory-leak caused by adding a new window resize handler
every time a popup is shown, but not removing it. This was causing
particularly bad behavior on IE, where a translucent glass element
allocates a significant amount of memory.
Review by: jlabanca (desk check)

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@7236 8db76d5a-ed1c-0410-87a9-c151d255dfc7