LibGUI: Add a GFrame class that can be inherited by framey widgets.

This will gather the code for painting sunken/raised frames etc in a single
place and make it easier add a bit of pleasant shading to UI's. :^)

LibGUI/GFrame.h

@@ -0,0 +1,33 @@
+#pragma once
+
+#include <LibGUI/GWidget.h>
+
+class GFrame : public GWidget {
+public:
+    explicit GFrame(GWidget* parent);
+    virtual ~GFrame() override;
+
+    enum Shadow { Plain, Raised, Sunken };
+    enum Shape { NoFrame, Box, Panel, VerticalLine, HorizontalLine };
+
+    int frame_thickness() const { return m_thickness; }
+    void set_frame_thickness(int thickness) { m_thickness = thickness; }
+
+    Shadow frame_shadow() const { return m_shadow; }
+    void set_frame_shadow(Shadow shadow) { m_shadow = shadow; }
+
+    Shape frame_shape() const { return m_shape; }
+    void set_frame_shape(Shape shape) { m_shape = shape; }
+
+    Rect frame_inner_rect() const { return rect().shrunken(m_thickness * 2, m_thickness * 2); }
+
+    virtual const char* class_name() const override { return "GFrame"; }
+
+protected:
+    void paint_event(GPaintEvent&) override;
+
+private:
+    int m_thickness { 0 };
+    Shadow m_shadow { Plain };
+    Shape m_shape { NoFrame };
+};