Refactor Cart into checkerboard table

Closes: #173 (closed)

Removes clear all feature from: #233

This one improves focus behavior in the cart, moving cart items and deleting items from the cart.

clive/__private/ui/operations/cart.py

@@ -308,8 +308,8 @@ class CartTable(CliveCheckerboardTable):
             return
 
         if removed_item.is_last:
-            second_last_cart_index = -2
-            cart_item_to_focus = cart_items[second_last_cart_index]
+            second_last_cart_item_index = -2
+            cart_item_to_focus = cart_items[second_last_cart_item_index]
         else:
             next_cart_item_index = removed_item.operation_index + 1
             cart_item_to_focus = cart_items[next_cart_item_index]