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/FormWidget.h

@@ -0,0 +1,22 @@
+#pragma once
+
+#include <LibGUI/GWidget.h>
+
+class FormEditorWidget;
+
+class FormWidget final : public GWidget {
+    C_OBJECT(FormWidget)
+public:
+    virtual ~FormWidget() override;
+
+    FormEditorWidget& editor();
+    const FormEditorWidget& editor() const;
+
+private:
+    virtual void paint_event(GPaintEvent&) override;
+
+    explicit FormWidget(FormEditorWidget& parent);
+
+    // FIXME: This should be an app-wide preference instead.
+    int m_grid_size { 5 };
+};