replace framebuffer.h include with forward declaration

instead of including framebuffer.h almost everywhere, replace it with
class CFrameBuffer forward declarations and/or generic system includes.
Add a hack to define fb_pixel_t to config.h (one reason for
framebuffer.h includes was the fb_pixel_t define)

src/system/helpers.cpp

@@ -47,6 +47,7 @@
 #include "debug.h"
 #include <global.h>
 #include <driver/fontrenderer.h>
+#include <driver/framebuffer.h>
 #include <system/helpers.h>
 #include <gui/update_ext.h>
 using namespace std;