Execute NSOperationQueue mainQueue on main thread

[triplef]

Currently [NSOperationQueue mainQueue] is just a normal queue, meaning it executes operations on a dedicated thread. However the expectation of this API is that operations get executed on the main thread, as is done with this change.

[triplef]

When building with libdispatch, NSOperationQueue should really be backed by libdispatch queues and support the underlyingQueue property (and fix this issue by using the libdispatch main queue as underlying queue for the mainQueue property), but that’s a bigger refactor (there’s an old branch that Greg had started for this here).

[rfm]

While I don't think that executing in the main thread is a good or intuitive behavior (to me the main point of an operation queue is to avoid blocking the main thread), I agree that it seems to be the way OSX implements it, and it's easy to work around by creating a new queue.

[triplef]

Thanks Richard. This API is usually used to make block-based APIs like this NSNotificationCenter API run the block on the main thread.