Mega change to the LCDd's header files:

1. Use <stdbool.h> if it is available but keep a compatibility shim in
   shared/defines.h in case it is not.
2. Unbreak the circular header dependencies (mostly):
   2a. client.c and client.h may NOT depend on 'struct Menu'. Use a void pointer
       and casts instead.
   2b. Separate the data type definitions in client.h and screen.h from
       function prototypes which may require other data structures. This makes
       header dependencies much more easy to maintain. Usually the data types
       should have been moved to their own files but I (mmdolze) chose to
       separate them using different header guards.
   2c. Remove many now unused header includes.
Apply (old BSD-) style on menuscreens.c.

server/widget.h

@@ -10,13 +10,12 @@
  * Copyright (c) 1999, William Ferrell, Selene Scriven
  */
 
-#include "screen.h"
-/* These headers are placed here on purpose ! (circular references) */
-
 #ifndef WIDGET_H
 #define WIDGET_H
 
-struct Widget;
+#define INC_TYPES_ONLY 1
+#include "screen.h"
+#undef INC_TYPES_ONLY
 
 /* These correspond to the index into the "types" array...*/
 typedef enum WidgetType {