[simple] checkboxes can get the focusable

Do note that checkboxes in the engine tab are displayed reversed.
See: https://github.com/searxng/searxng/blob/3408d061aab9abc6168fec9bbc6deab71b236dac/searx/templates/simple/preferences.html#L313
A checkbox for an engine is checked when the engine is disabled.

searx/static/themes/simple/src/less/animations.less

@@ -13,3 +13,7 @@
     }
   }
 }
+
+input.checkbox-onoff[type="checkbox"]::before {
+  transition: left 0.25s;
+}