IE plugin now uses the current thread ID as a proxy for tab ID.

This seems to work well for IE6 and IE7.  Still need to test IE8.

Review by: jat (desk)

git-svn-id: https://google-web-toolkit.googlecode.com/svn/branches/farewellSwt@6256 8db76d5a-ed1c-0410-87a9-c151d255dfc7