InputCommon/WGInput: Handle add/remove events on separate thread to prevent deadlocks.

[AdmiralCurtiss]

In particular this is triggered when running Dolphin with the Steam overlay.

This should be a more formally correct alternative to #12504. Built on top of #12503 so merge that first please.

[Filoppi]

So this will be run in a separate thread, but when calling this:

s_event_added = WGI::RawGameController::RawGameControllerAdded(
  [](auto&&, WGI::RawGameController raw_game_controller) {
    s_device_add_remove_queue.EmplaceItem(
        AddRemoveEvent{AddRemoveEventType::Add, std::move(raw_game_controller)});
        });

is it going to wait for this thread to have run the event, or will it be delayed to later? If it's done "inline" then there's still a risk of deadlock.

[AdmiralCurtiss]

Nothing should block until completion here. WGI::RawGameController::RawGameControllerAdded() registers a lambda to be called every time a new RawGameController is noticed by WGI. s_device_add_remove_queue.EmplaceItem() registers an item to be executed in the WorkQueueThread's worker thread and does not wait for that item to be processed or anything like that -- it may block very shortly while the item is added into the queue if another thread is also adding to the queue or while the worker thread is taking an item from the queue to to be executed, but that can never cause a deadlock.