add demo application (benchmark)

examples/demo/app.css

@@ -0,0 +1,37 @@
+.main {
+  position: absolute;
+  left: 0;
+  right: 0;
+  top: 0;
+  bottom: 0;
+
+  display: grid;
+  grid-template-columns: 200px 1fr;
+}
+
+.left-thing {
+  background-color: gray;
+  padding: 20px;
+}
+
+.left-thing button {
+  width: 100%;
+}
+
+.right-thing {
+  padding: 20px;
+  overflow: auto;
+}
+
+/* Message widget */
+.message .author {
+  font-weight: bold;
+}
+
+.message {
+  width: 400px;
+  background-color: lightblue;
+  margin: 10px 5px;
+  border-radius: 5px;
+  padding: 5px;
+}