blackbox: deprecate package due to dead upstream

There has been no Blackbox release since 2005, which indicates that
the upstream project is dead. Since there are many other maintained
and alive window manager projects around, I'd like to nominate
Blackbox for deprecation.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>