Changing the list formfield to remove the class "advancedSelect" if there are more than 10'000 options since Chosen has performance issues with so many options.

Adding "advancedSelect" class as a default class to list fields so extensions don't have to change anything if they have no classes specified themself.
Changing the menu item form to use the class "advancedSelect" so Chosen is explicitely allowed on this elements.
Set the formbehaviour.chosen to only act on selects with the CSS class "advancedSelect" instead on all selects.
This is the default behaviour for this function.