Fixes Issue #312.
Changes PopupPanel to override onDetach() to guarantee that
DOM.removeEventPreview() will be called, regardless of the mechanism used
to remove the PopupPanel from the document.

Patch by: bobv
Review by: knorton



git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@991 8db76d5a-ed1c-0410-87a9-c151d255dfc7