Added a NativePreviewEvent to replce the current EventPreview system. Now, multiple widgets can preview the same event, instead of just the one at the top of the stack. This fixes a couple of subtle bugs and adds some use cases, such as listening for user timeout. Patch by: jlabanca Review by: ecc Issue: 1275 git-svn-id: https://google-web-toolkit.googlecode.com/svn/releases/1.6@4385 8db76d5a-ed1c-0410-87a9-c151d255dfc7