nats in Haskell are readable

Let rsp and prs in fun_rel/fun_map format

tuned zoom_box; tuned tooltips;

print calculation result in the context where the fact is actually defined -- proper externing; misc tuning;

future_job: propagate current Position.thread_data to the forked job -- this is important to provide a default position, e.g. for parallelizied Goal.prove within a package (proper command transactions are wrapped via Toplevel.setmp_thread_position);

some message styling;

simplified message markup, using plain XML.Elem directly;

more robust Position.setmp_thread_data, independently of Output.debugging (essentially reverts f9ec18f7c0f6, which was motivated by clean exception_trace, but without transaction positions the Isabelle_Process protocol breaks down);

refrain from forcing a hardwired SHELL value, cf. 1494ded298a6 but it becomes obsolete again in 549969a7f582 and follow-ups;

bad_result: report fully explicit message;