Changing items when default ones don't work

[Lemvs]

The ability to set another item in a slot when the current one doesn't work, for example if I'm on the first page I can't go back to the previous one, so an option like "else-item: X" would be useful, where after else we provide the item created earlier for this menu, which is to be displayed in case the default one doesn't work, this would allow us to remove items from pages when we're on the first/last page or there are no pages at all (such an option should exist for items of the previous and next pages, for the back item and for the slots item).

For example, if the previous page (%) does not exist, the item in this slot will be replaced with the item $