Adding @defgroup and @ingroup doxygen tags into all headers
in the common folder that contain doxygen blocks.
This improves the structure, readability, and findability
of information in the resulting output.
This commit targets purely structure and does not deal with
the content of the currently existing doxygen documentation.
This also removes the dependency of engines on the event recorder header
and API, and will make it easier to RandomSources that are not properly
registered.
