Add a prefetch queue to AsyncFragmentLoader.  User code can add split points to
the queue, and AsyncFragmentLoader will load the code for them whenever it has
nothing else to do.

Also updates the Showcase sample to prefetch code within sample categories.  For
example, clicking on any of the "panels" samples causes all of the other panels
samples' code to be prefetched.

Review by: bobv



git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@6992 8db76d5a-ed1c-0410-87a9-c151d255dfc7