[mod] replace engines_languages.json by engines_traits.json

Implementations of the *traits* of the engines.

Engine's traits are fetched from the origin engine and stored in a JSON file in
the *data folder*.  Most often traits are languages and region codes and their
mapping from SearXNG's representation to the representation in the origin search
engine.

To load traits from the persistence::

    searx.enginelib.traits.EngineTraitsMap.from_data()

For new traits new properties can be added to the class::

    searx.enginelib.traits.EngineTraits

.. hint::

   Implementation is downward compatible to the deprecated *supported_languages
   method* from the vintage implementation.

   The vintage code is tagged as *deprecated* an can be removed when all engines
   has been ported to the *traits method*.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

docs/admin/engines/configured_engines.rst

@@ -42,7 +42,7 @@ Explanation of the :ref:`general engine configuration` shown in the table
         - Timeout
         - Weight
         - Paging
-        - Language
+        - Language, Region
         - Safe search
         - Time range
 

docs/admin/engines/settings.rst

@@ -569,10 +569,13 @@ engine is shown.  Most of the options have a default value or even are optional.
   To disable by default the engine, but not deleting it.  It will allow the user
   to manually activate it in the settings.
 
+``inactive``: optional
+  Remove the engine from the settings (*disabled & removed*).
+
 ``language`` : optional
   If you want to use another language for a specific engine, you can define it
-  by using the full ISO code of language and country, like ``fr_FR``, ``en_US``,
-  ``de_DE``.
+  by using the ISO code of language (and region), like ``fr``, ``en-US``,
+  ``de-DE``.
 
 ``tokens`` : optional
   A list of secret tokens to make this engine *private*, more details see