STM: tell the user if the master is recomputing states validated by workers

When the worker fails, the master may need to recompute some states
the worker has already validates.  In this case they are colored
accordingly.
STM: worker sends back to master the last valid state

So that the master process does not require to compute it.
Still not all valid states are sent back.
Work around for bug in threads + blocking io streamlined
STM + CoqIDE: stop_worker message and UI
Load implemented in CoqIDE/STM (closes: #3223)