Fix deadlock in multithreaded excerpt serialisation

[ajuncosa]

The goal is to avoid side effects on the score during write/serialisation, so excerpt serialisation can safely run in separate threads.

There was a deadlock when saving scores that contained multi measure rests and some hidden parts. The save called layout in that case, which frees Systems/Pages, and the EngravingItem destructor calls Score::onElementDestruction which ends with score->elementDestroyed().send(e). This send was cross-thread, and that channel has a main-thread receiver for every score, so the send must register a port on the main thread's slot (QueuePool::regPort), and this locks the same mutex that the main thread is holding while it's processing queued callbacks (in QueuePool::processMessages), and the save is one of the queued commands.

Apart from the freezing doLayout() call, the write was also modifying the score in other ways (start/endCmd(), undoChangeProperty(VISIBLE, true)), and all this routes to the master score's single shared TransactionManager, whose endTransaction calls m_masterScore->update(), relaying out the master from a worker thread.

I have moved the unhide and relayout out of the writer.