[release,entityseed] Added isValid and default constructor.

In revisions d6d87aafdd48d39e4c50403e04821c5c98f8e6a9, and
4974aa47828af5da96a7f1ba576bd47f21816d4e of dune-grid the interface
of entityseed was altered. It now has to be default constructible and
have method isValid. This patch applies the necessary changes to adhere
to this interface and make check/test succeeds again.
(cherry picked from commit cdf66ed0ab6631875cf0b3c0cdf6543bab156ea4)

Signed-off-by: Markus Blatt <markus@dr-blatt.de>