Revert "Disable threaded saving as a workaround for stall during quit"

This reverts commit 0b419474120345dd71095d05ea412ac33c7b85c6.
Revert "Do not abort application shutdown when we need to save documents"

This reverts commit 73f82c6c5ccb6d61a92c2786545fa7b6346d6833.

Conflicts:
	Frameworks/DocumentWindow/src/DocumentController.mm
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