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.

Demos/Host/LowLevel/KeyboardHostWithParser/HIDReport.h

@@ -41,6 +41,10 @@
 
 		#include "KeyboardHostWithParser.h"
 
+	/* Macros: */
+		/** HID Report Descriptor Usage Page value for a desktop keyboard. */
+		#define USAGE_PAGE_KEYBOARD             0x07
+
 	/* Enums: */
 		/** Enum for the possible return codes of the \ref GetHIDReportData() function. */
 		enum KeyboardHostWithParser_GetHIDReportDataCodes_t