subscription as target-specific implementation device

make SML/NJ happier;

merged

updated properties for scala.concurrent.ExecutionContext.Implicits.global (future task farm), similar to Isabelle/ML; removed obsolete properties for old actors;

simplified change_buffer (again, see 937826d702d5): no thread, just timer, rely on asynchronous commands_changed.post;

replaced manager Actor by Consumer_Thread, which is lazy to defer its start to actual Handler init time;

tuned whitespace;

obsolete;