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/DemoWizardPage1.gml

@@ -0,0 +1,37 @@
+@GUI::Widget {
+    layout: @GUI::VerticalBoxLayout {
+        margins: [20, 20, 20, 20]
+    }
+
+    @GUI::Label {
+        text: "Please select an installation directory."
+        text_alignment: "TopLeft"
+        fixed_height: 32
+    }
+
+    @GUI::Widget {
+        fixed_height: 25
+
+        layout: @GUI::HorizontalBoxLayout {
+        }
+
+        @GUI::Label {
+            text: "Location: "
+            autosize: true
+        }
+
+        @GUI::TextBox {
+            name: "page_1_location_text_box"
+        }
+
+        @GUI::Button {
+            text: "Browse"
+            fixed_width: 75
+        }
+    }
+
+    // Spacer
+    @GUI::Widget {
+
+    }
+}