GMLPlayground: Move layout to GML

There isn't much layout right now, but it felt very wrong to not have
the GML editor use GML. :^)

Userland/DevTools/GMLPlayground/GMLPlaygroundWindow.gml

@@ -0,0 +1,17 @@
+@GUI::Widget {
+    layout: @GUI::VerticalBoxLayout {}
+    fill_with_background_color: true
+
+    @GUI::HorizontalSplitter {
+        layout: @GUI::HorizontalBoxLayout {}
+        name: "splitter"
+
+        @GUI::TextEditor {
+            name: "text_editor"
+        }
+
+        @GUI::Frame {
+            name: "preview_frame"
+        }
+    }
+}