[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/preferences.py

@@ -17,7 +17,7 @@ from searx.enginelib import Engine
 from searx.plugins import Plugin
 from searx.locales import LOCALE_NAMES
 from searx.webutils import VALID_LANGUAGE_CODE
-from searx.engines import OTHER_CATEGORY
+from searx.engines import DEFAULT_CATEGORY
 
 
 COOKIE_MAX_AGE = 60 * 60 * 24 * 365 * 5  # 5 years
@@ -259,7 +259,7 @@ class EnginesSetting(BooleanChoices):
         choices = {}
         for engine in engines:
             for category in engine.categories:
-                if not category in list(settings['categories_as_tabs'].keys()) + [OTHER_CATEGORY]:
+                if not category in list(settings['categories_as_tabs'].keys()) + [DEFAULT_CATEGORY]:
                     continue
                 choices['{}__{}'.format(engine.name, category)] = not engine.disabled
         super().__init__(default_value, choices)