Allow handling order changes on order level instead of in the cart

Currently commerce assumes that an order can only be changed while in the cart. An example of this is that
the checkout step is only reset from the CartManager::resetCheckoutStep(), but also all events emitted related to changes to carts content. Any change on the order content from the order entity does nothing. (eg. $order->addItem()

This does not really make sense because a cart is not even a special entity. It is only an order with a boolean value. For consistency (and contrib) I strongly believe it would make more sense to handle all these events in the order entity. Besides, you can install commerce_order without installing commerce_cart. In this situation, you have no way to know what happens in an order.

Proposed resolution

- Move (and deprecate) all cart events to commerce_order and emit them from there; optionally with a cart parameter.
- Rework all cart events and change them into order events. If action is needed only for a cart; check $event->isCart() or $event->getOrder->get('cart')->value.

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Original issue

There are no events for altering [adding/updating/deleting] an order's contents (order items) for alterations that happen outside of a cart. For some event handlers it makes sense to subscribe to events where an order item is added/updated/deleted on any order instead of only for cart orders. This became evident in #3083384: Checkout flows should be resolved again after the cart/order has changed.

Proposal: Have Order::addItem(), Order::removeItem() and Order::setItems() also emit events to let other modules know that the order's items have changed. These should be similar to the events emitted by CartManager.

Optional: Copy the Cart::updateItem() to Order to have a consistent behavior and a central place for emitting events. (It can be called from the cart manager.)