[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/plugins/vim_hotkeys.py

@@ -1,10 +0,0 @@
-from flask_babel import gettext
-
-name = gettext('Vim-like hotkeys')
-description = gettext(
-    'Navigate search results with Vim-like hotkeys '
-    '(JavaScript required). '
-    'Press "h" key on main or result page to get help.'
-)
-default_on = False
-preference_section = 'ui'