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/Common/MIDI.h

@@ -87,6 +87,7 @@
 		#define MIDI_CHANNEL(channel)        ((channel) - 1)
 
 	/* Enums: */
+		/** Enum for the possible MIDI jack types in a MIDI device jack descriptor. */
 		enum MIDI_JackTypes_t
 		{
 			MIDI_JACKTYPE_Embedded = 0x01, /**< MIDI class descriptor jack type value for an embedded (logical) MIDI input or output jack. */