[mod] clarify the difference of the default category and subgrouping

This PR does no functional change it is just an attempt to make more clear in
the code, what a default category is and what a subcategory is.  The previous
name 'others' leads to confusion with the **category 'other'**.

If a engine is not assigned to a category, the default is assigned::

    DEFAULT_CATEGORY = 'other'

If an engine has only one category and this category is shown as tab in the user
interface, this engine has no further subgrouping::

    NO_SUBGROUPING = 'without further subgrouping'

Related:

- https://github.com/searxng/searxng/issues/1604
- https://github.com/searxng/searxng/pull/1545

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

searx/engines/__init__.py

@@ -45,7 +45,7 @@ ENGINE_DEFAULT_ARGS = {
     "about": {},
 }
 # set automatically when an engine does not have any tab category
-OTHER_CATEGORY = 'other'
+DEFAULT_CATEGORY = 'other'
 
 
 # Defaults for the namespace of an engine module, see :py:func:`load_engine`
@@ -132,7 +132,7 @@ def load_engine(engine_data: dict) -> Optional[Engine]:
     set_loggers(engine, engine_name)
 
     if not any(cat in settings['categories_as_tabs'] for cat in engine.categories):
-        engine.categories.append(OTHER_CATEGORY)
+        engine.categories.append(DEFAULT_CATEGORY)
 
     return engine