Added wayland discrete axis events for mouse wheel

the discrete axis event gives changes in steps just like the
xfreerdp version uses. This way scrolling can be implemented
consistent with the behaviour of xfreerdp

uwac/include/uwac/uwac.h

@@ -107,6 +107,7 @@ enum
 	UWAC_EVENT_CLIPBOARD_SELECT,
 	UWAC_EVENT_CLIPBOARD_OFFER,
 	UWAC_EVENT_OUTPUT_GEOMETRY,
+	UWAC_EVENT_POINTER_AXIS_DISCRETE
 };
 
 /** @brief window states */