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/driver/pictureviewer/gif.cpp

@@ -1,3 +1,4 @@
+#include <config.h>
 #include "pv_config.h"
 
 #include <cstring>