Add include directories to library target
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This properly sets up the dune-foamgrid main target to follow modern CMake practices where the target holds all the information to propagate transitively to other targets. The MR also suppresses the warning that a Dune policy is not set, by setting it to NEW. The consequence is that there is no global include_directories(...) call in dune_project anymore and one needs to link all targets that need Dune include directories either to the library target Dune::FoamGrid (or dunefoamgrid) or set the include directories manually, e.g., using the utility function dune_default_include_directories(<target> [PUBLIC|PRIVATE|INTERFACE]). This is especially important for tests. That is why tests should be linked against the library target.