ThemeEditor: Convert the widget-preview layout to GML

This has the nice bonus of fixing the appearance of the status bar. :^)

Userland/Applications/ThemeEditor/Previews/WindowPreview.gml

@@ -0,0 +1,29 @@
+@GUI::Frame {
+    layout: @GUI::VerticalBoxLayout {}
+
+    @GUI::Widget {
+        layout: @GUI::VerticalBoxLayout {
+            margins: [8, 100, 8, 8]
+        }
+
+        @GUI::Button {
+            text: "Button"
+        }
+
+        @GUI::CheckBox {
+            text: "Check box"
+        }
+
+        @GUI::RadioButton {
+            text: "Radio button"
+        }
+
+        @GUI::TextEditor {
+            text: "Text editor\nwith multiple\nlines."
+        }
+    }
+
+    @GUI::Statusbar {
+        text: "Status bar"
+    }
+}