LibGUI: Add a GPainter class that inherits from Painter.

This gets rid of the last little piece of LibGUI knowledge in Painter.

LibGUI/GPainter.h

@@ -0,0 +1,12 @@
+#pragma once
+
+#include <SharedGraphics/Painter.h>
+
+class GWidget;
+class GraphicsBitmap;
+
+class GPainter : public Painter {
+public:
+    explicit GPainter(GWidget&);
+    explicit GPainter(GraphicsBitmap&);
+};