[cmake] major change in GridUtils
* we add the include dirs and link dirs per grid type
  depending only on the fact, that the grid was found
* we also add the libs of all found grids to GRIDLIBS
* depending on the users choice of GRIDTYPE we only
  set stuff like -DENABLE_FOO for the
  Dune::GridSelector, nothing else
All in all, this should finally allow to use different
grids in one place (e.g. lib).
[grid.fakeentity] added FakeEntity< dim >
* only provides type() atm
[functions.default] we do not need this include atm