Add animation when an item is added to the list

gui/ui/state.slint

@@ -2,6 +2,10 @@ export struct SItem {
     name: string,
     checked: bool,
     amount: int,
+
+    /// Whether the item was just added by the user.
+    /// Determines whether the add-animation plays.
+    just_added: bool,
 }
 
 export struct SButtonItem{