Drop outdated engine attributes: supported_languages, language_aliases
Complete, normalize and document the type definitions for the engine-module and
engine-class.
For the ``engine.about`` section of the configuration, a type check is performed
based on structure ``searx.enginelib.EngineAbout``.
The property ``engine.about.language`` no longer exists; existing values have
been migrated to ``engine.language``.
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>
Bnyro