add router service, better structure for demo app

demo/src/widgets/Counter.ts

@@ -0,0 +1,26 @@
+import Widget from "../../../src/core/widget";
+
+const template = `
+    <div>
+        <button t-on-click="increment(-1)">-</button>
+        <span style="font-weight:bold">Value: <t t-esc="state.counter"/></span>
+        <button t-on-click="increment(1)">+</button>
+    </div>
+`;
+
+export default class Counter extends Widget {
+  name = "counter";
+  template = template;
+  state = {
+    counter: 0
+  };
+
+  constructor(parent: Widget | null, props: {initialState?: number}) {
+    super(parent);
+    this.state.counter = props.initialState || 0;
+  }
+
+  increment(delta: number) {
+    this.updateState({ counter: this.state.counter + delta });
+  }
+}