Move LibGUI/GStyle to SharedGraphics/StylePainter.

I want to paint some buttons in WindowServer where we don't have LibGUI.

SharedGraphics/StylePainter.h

@@ -0,0 +1,17 @@
+#pragma once
+
+class Painter;
+class Rect;
+
+enum class ButtonStyle { Normal, CoolBar, OldNormal };
+
+class StylePainter {
+public:
+    static StylePainter& the();
+
+    void paint_button(Painter&, const Rect&, ButtonStyle, bool pressed, bool hovered = false);
+    void paint_surface(Painter&, const Rect&);
+
+private:
+    StylePainter();
+};