LibGUI: Add a simple GGroupBox widget.

This needs some work on the visual side, but it gets the job done already.

LibGUI/GGroupBox.h

@@ -0,0 +1,19 @@
+#pragma once
+
+#include <LibGUI/GWidget.h>
+
+class GGroupBox : public GWidget {
+public:
+    GGroupBox(const String& name, GWidget* parent);
+    virtual ~GGroupBox() override;
+
+    String name() const { return m_name; }
+
+    virtual const char* class_name() const override { return "GGroupBox"; }
+
+protected:
+    virtual void paint_event(GPaintEvent&) override;
+
+private:
+    String m_name;
+};