[cmake] Modernize examples/CMakeLists.txt

Replace target_link_dune_default_libraries(target) by the modern dune_target_enable_all_packages(target). The problem with the former is, that it does not append definitions, such that we cannot use UGGrid because ENABLE_UG is not defined. Surprisingly this is in contrast to dune_add_test(...) which does add the definitions.

Alternatively we could call dune_enable_all_packages() in our root CMakeLists.txt, but I'm unsure about the implications.

Notice that this reveals several open questions regarding dune-common:

• Should target_link_dune_default_libraries() still be used or should it be deprecated in favour of dune_target_enable_all_packages()?
• Should the src/ example created by duneproject still call target_link_dune_default_libraries() altough dune_enable_all_packages() is called by default?
• Should the call to add_dune_all_flags() in dune_add_test() be replaced by dune_target_enable_all_packages() such that tests are build similarly to normal executables?
• Should a library module (like, e.g. dune-functions) use dune_enable_all_packages()? Or is this discouraged, because it has to be used in all derived modules?
• Do we allow building anything without using one of the add-all-flags approaches at all? In view of possible binary incompatibility it looks like "All tests use it" implies "All libraries have to use it" which implies "All applications have to use it"