Fix the cross-thread message queue to actually act as a queue, not a stack

[llemoi]

The message queue used (only?) to display messages in the GUI script console panel uses a std::deque internally, and was putting messages into it and taking them out from the same end.

When messages were put into to the queue faster than they were taken out, it was therefore acting as a stack and printing batches of them in reverse order.

[haganbmj]

Neat catch, thanks for grabbing that and sorry it took me a while to get around to it.