Code changed a bit to make it work as before with OCaml 4.0.

Code changed a bit to make it work as before with OCaml 4.0.
Without this patch, the refresh of the textual widget (e.g. the update
of the locked part) is not immediately done.

This is roughly the visual behaviour the user expects. On the other hand,
we also expect the threads that execute and that responsible for the UI
to work in parallel, which is not the case. The code I just touched stops
the worker thread for, on average, 0.1s per tactic, which is really a lot.
Switching to a multiprocess implementation (in place of a multithread) does
not seem easy. Bad OCaml, bad.