Add descriptor class, subclass and protocol constants to the class drivers, modify all demos to use them where possible.

Move out private/internal host class driver constants to the common class driver headers, so that they can be used in the Low Level host mode demos.

Ensure all demos, projects and bootloaders use the class driver constants where possible to minimise code repetition.

LUFA/Drivers/USB/Class/Host/HID.h

@@ -297,9 +297,6 @@
 
 	/* Private Interface - For use in library only: */
 	#if !defined(__DOXYGEN__)
-		/* Macros: */
-			#define HID_INTERFACE_CLASS             0x03
-
 		/* Function Prototypes: */
 			#if defined(__INCLUDE_FROM_HID_HOST_C)
 				static uint8_t DCOMP_HID_Host_NextHIDInterface(void* const CurrentDescriptor) ATTR_NON_NULL_PTR_ARG(1);