Initial port, including manual and tests (still wip).

Known issues:

 * mkhtml2.py unit-tests module  relies on lxml, anytree and pygments
   but these dependencies are not track by the build system.

 * Integration tests documentation generation are broken.

 * gtkdoc_scangobj_runner.py builds --cflags and --libs itself in a
   non-portable (and wrong) way.

https://bugzilla.gnome.org/show_bug.cgi?id=794087