tag: add a refresh_in() interface function

E.g. particles may use this to force a refresh after a certain amount
of time.

Note that it can only be used with 'realtime' tags.

tag.h

@@ -25,6 +25,8 @@ struct tag {
     long (*min)(const struct tag *tag);
     long (*max)(const struct tag *tag);
     enum tag_realtime_unit (*realtime)(const struct tag *tag);
+
+    bool (*refresh_in)(const struct tag *tag, long milli_seconds);
 };
 
 struct tag_set {