Show_script called only if in coqtop mode
STM: reorganize code and file names

- proofworkertop to deal with proof tasks
- tacworkertop   to deal with par: tactics
- queryworkertop to deal with queries (next commit)
STM: new worker for queries

With the options -async-queries-always-delegate queries are
always delegated to a worker process (Eval, Check, ...).

Users of PIDE based UIs (in Denmark) reported that the current
behavior of processing query synchronously is rather unexpected
when one is used to get proofs processed asynchronously.

Non instantaneous queries are part of many scripts and are there
as "tests" for testing the execution of recursive functions.
A standard proof script shape in an ongoing work by Appel and Bengtson
is made of blocks like:
- recursive function definition,
- some tests,
- some proofs
And one cannot quickly jump over the tests (only the proofs).
Enclosing the queries into dummy proofs to recover a reactive UI
is just annoying.  Hence this patch.

Currently CoqIDE is not able to integrate the asynchronous feedback
of the query workers into the document, hence if one passes the option
to CoqIDE one only gets a boolean out of queries (processed/error).
Feedback message: hold extra info to help routing

PIDE based GUIs can take advantage of multiple panels and get
some feedback routed there. E.g. query panel