Demos: Add a HelloWorld2 demo.

This is a simple test app with its UI generated from a VisualBuilder form.
The name is probably silly, but who cares. :^)

Demos/HelloWorld2/main.cpp

@@ -0,0 +1,26 @@
+#include <LibGUI/GApplication.h>
+#include <LibGUI/GBoxLayout.h>
+#include <LibGUI/GButton.h>
+#include <LibGUI/GLabel.h>
+#include <LibGUI/GWidget.h>
+#include <LibGUI/GWindow.h>
+#include "UI_HelloWorld2.h"
+
+int main(int argc, char** argv)
+{
+    GApplication app(argc, argv);
+
+    auto* window = new GWindow;
+    window->set_rect(100, 100, 240, 160);
+    window->set_title("Hello World!");
+
+    auto* ui = new UI_HelloWorld2;
+    window->set_main_widget(ui->main_widget);
+
+    ui->w3->on_click = [&](auto&) {
+        app.quit();
+    };
+
+    window->show();
+    return app.exec();
+}