[enh] introduce categories_as_tabs

Previously all categories were displayed as search engine tabs.
This commit changes that so that only the categories listed under
categories_as_tabs in settings.yml are displayed.

This lets us introduce more categories without cluttering up the UI.
Categories not displayed as tabs  can still be searched with !bangs.

searx/engines/__init__.py

@@ -13,6 +13,7 @@ usage::
 
 import sys
 import copy
+import itertools
 
 from os.path import realpath, dirname
 from babel.localedata import locale_identifiers
@@ -260,3 +261,26 @@ def load_engines(engine_list):
         if engine:
             register_engine(engine)
     return engines
+
+
+DEFAULT_GROUP_NAME = 'others'
+
+
+def group_engines_in_tab(engines):  # pylint: disable=redefined-outer-name
+    def engine_sort_key(engine):
+        return (engine.about.get('language', ''), engine.name)
+
+    def group_sort_key(group):
+        return (group[0] == DEFAULT_GROUP_NAME, group[0].lower())
+
+    def get_group(eng):
+        non_tab_engines = [c for c in eng.categories if c not in settings['categories_as_tabs']]
+        return non_tab_engines[0] if len(non_tab_engines) > 0 else DEFAULT_GROUP_NAME
+
+    return [
+        (groupname, sorted(engines, key=engine_sort_key))
+        for groupname, engines in sorted(
+            ((name, list(engines)) for name, engines in itertools.groupby(sorted(engines, key=get_group), get_group)),
+            key=group_sort_key,
+        )
+    ]