LibGUI: Share code for widget rendering styles in a GStyle class.

Since GScrollBar wants its internal buttons to look like GButtons,
let's share the painting code between them.

LibGUI/GStyle.h

@@ -0,0 +1,14 @@
+#pragma once
+
+class Painter;
+class Rect;
+
+class GStyle {
+public:
+    static GStyle& the();
+
+    void paint_button(Painter& painter, const Rect& rect, bool pressed);
+
+private:
+    GStyle();
+};