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/gui/widget/stringinput_ext.h

@@ -35,12 +35,12 @@
 
 #include "menue.h"
 
-#include <driver/framebuffer.h>
 #include <system/localize.h>
 
 #include <string>
 #include <vector>
 
+class CFrameBuffer;
 class CExtendedInput_Item;
 class CExtendedInput : public CMenuTarget
 {