HackStudio: Start fleshing out the GUI for a GUI designer :^)

I'll be reconstructing parts of the VisualBuilder application here and
then we can retire VisualBuilder entirely once all the functionality
is available in HackStudio.

DevTools/HackStudio/FormEditorWidget.h

@@ -0,0 +1,18 @@
+#pragma once
+
+#include <LibGUI/GScrollableWidget.h>
+
+class FormWidget;
+
+class FormEditorWidget final : public GScrollableWidget {
+    C_OBJECT(FormEditorWidget)
+public:
+    virtual ~FormEditorWidget() override;
+
+private:
+    virtual void paint_event(GPaintEvent&) override;
+
+    explicit FormEditorWidget(GWidget* parent);
+
+    RefPtr<FormWidget> m_form_widget;
+};