Make sure we unregister for clipboard notifications on exit
on Windows. This is necessary because Windows doesn't implicitly
clean up when a process dies, and we cannot trust applications to
always explicitly unregister.


git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10367 ea41ed52-d2ee-0310-a9c1-e6b18d33e121