[IMP] store: prevent changes to store state in useStore return value

closes #500

tests/store_hooks.test.ts

@@ -100,6 +100,24 @@ describe("connecting a component to store", () => {
     expect(error.message).toBe("No store found when connecting 'App'");
   });
 
+  test("cannot modify state returned by usestore", async () => {
+    const state = { a: { b: 1 } };
+    const actions = {};
+    const store = new Store({ state, actions });
+
+    class App extends Component<any, any> {
+      static template = xml`<div/>`;
+      storeState = useStore(state => state.a);
+    }
+
+    (<any>env).store = store;
+    const app = new App();
+    expect(app.storeState.b).toBe(1);
+    expect(() => (app.storeState.b = 2)).toThrow(
+      "Store state should only be modified through actions"
+    );
+  });
+
   test("can use useStore twice in a component", async () => {
     const state = { a: 1, b: 2 };
     const actions = {