eina_cxx: add documentation to the Eina C++ header files.

eina_cxx: add documentation to the Eina C++ header files.

Summary:
Added documentation to almost all classes that are intended for direct use by the library users.
Marked classes/functions used by the library itself as @internal.
Modified the Doxyfile.in to enable the generation of documentation to the bindings.

Reviewers: felipealmeida, cedric, woohyun, smohanty, raster

CC: savio, cedric
Differential Revision: https://phab.enlightenment.org/D947

Signed-off-by: Cedric BAIL <c.bail@partner.samsung.com>