[mod] preferences: implement drop-down menu for hotkeys (default, vim)

Replace the on/off checkbox of the vim-hotkeys in the preferences by a drop-down
menu.

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

searx/preferences.py

@@ -465,6 +465,10 @@ class Preferences:
                 settings['ui']['search_on_category_select'],
                 locked=is_locked('search_on_category_select')
             ),
+            'hotkeys': EnumStringSetting(
+                'default',
+                choices=['default', 'vim']
+            ),
             # fmt: on
         }