pgp: fix the pgp namespace
In the rest of the codebase, we assume that each module provides its own
namespace under the general Seahorse
namespace (for example,
Seahorse.Ssh
). In the Pgp module however, objects were directly under
the Seahorse namespace (Seahorse.PgpBackend
vs Seahorse.Pgp.Backend
)
which gave issues once we tried to let the Vala code and the C code
cooperate.