WidgetGallery: add a simple Wizard demo :^)

The sample Wizard subclasses WizardDialog and demonstrates a front and
back cover, as well as extracting user input from a Wizard page to
display in the interface which spawned the Wizard.

Userland/Demos/WidgetGallery/DemoWizardPage2.gml

@@ -0,0 +1,21 @@
+@GUI::Widget {
+    layout: @GUI::VerticalBoxLayout {
+        margins: [20, 20, 20, 20]
+    }
+
+    @GUI::Label {
+        text: "Please wait..."
+        text_alignment: "TopLeft"
+        fixed_height: 32
+    }
+
+    @GUI::ProgressBar {
+        name: "page_2_progress_bar"
+        fixed_height: 28
+    }
+
+    // Spacer
+    @GUI::Widget {
+
+    }
+}