Add add_all to Sequence

src/lib/Concrete/Sequence.vala

@@ -53,6 +53,10 @@ namespace Invercargill {
             }
         }
 
+        public void add_all(Enumerable<T> items) {
+            items.iterate(i => add(i));
+        }
+
         public override int count() {
             return n_items;
         }