Put handling of engine output in separate routine

Put handling of engine output in separate routine

The reading, decoding and handling of the engine output is now put in
a routine HandleEngineOutput(), which is still an infinite loop that
wakes up a blocked GUI thread after a 'uciok', 'readyok' or 'bestmove'
command. This in preparation of a new design, where it would break out
of the loop and return after these commands, to let its caller process
any queued GUI commands.