rename the Weak stuff in tools/ref.hxx to WeakRef

.. which more accurately reflects it's purpose

Change-Id: Ibb87a47a3d1f3e35ac2820f14c71adf3b227c961
Reviewed-on: https://gerrit.libreoffice.org/10283
Reviewed-by: Thomas Arnhold <thomas@arnhold.org>
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>