Create template aliases to simplify usage of BackendVectorSelector etc.

Using template aliases, we can turn

typedef typename Dune::PDElab::BackendVectorSelector<GFS,E>::Type Vector;

into

typedef Dune::PDELab::Backend::Vector<GFS,E> Vector;
// or, even nicer:
using Vector = Dune::PDELab::Backend::Vector<GFS,E>;

which is a lot more readable. Something similar can be done for the matrix equivalent