When `CGAL_HEADER_ONLY` and `WITH_{examples|tests|demos}`, then only
the first call to `find_package(CGAL)` does the job. The subsequent
calls return very fast, by caching the results in global properties.
Now the variables `CGAL_Qt5_MOC_FILES` and `CGAL_Qt5_RESOURCE_FILES` are no
longer used by the definition of `CGAL_Qt5_extras`: other names are used.
`CGAL_Qt5_extras` is not supposed to be used directly by
users/examples/demos' `CMakeLists.txt`.
- Now even CGAL_Core has one.
- The `${cgal_lib}_FOUND` are now set in those modules, after having chec
- For self-consistency, `Use_CGAL_Qt5_headers` is included by
`CGAL_SetupCGAL_Qt5Dependencies`.