Unify item modification

[seionmoya]

In

• RagFairBuyOfferItemEventController
• TradingConfirmEventController

The following gets repeated:

if (itemInstance.Updatable.StackObjectsCount <= 0)
{
    profile.Pmc.Inventory.RemoveItem(itemInstance);
    context.Response.ProfileChanges[profile.Pmc._id].Items.Delete.Add(itemInstance);
}
else
{
    context.Response.ProfileChanges[profile.Pmc._id].Items.Change.Add(itemInstance);
}

var stacks = ItemFactoryService.Instance.CreateItemsFromTradeRequest(itemsToBuy, request.Count);

foreach (var stack in stacks)
{
    (int itemWidth, int itemHeight) = _itemService.CalculateItemSize(stack);
    var targetLocation = profile.Pmc.Inventory.GetNextFreeSlot(_itemService, itemWidth, itemHeight, out string gridName);

    if (targetLocation == null)
    {
        throw new Exception("No room for item");
    }

    var rootItem = stack[0];
    rootItem.Location = targetLocation;
    rootItem.SlotId = gridName;
    rootItem.ParentId = profile.Pmc.Inventory.Stash;

    profile.Pmc.Inventory.AddItems(_itemService, ItemFactoryService.Instance,
        stack);

    context.Response.ProfileChanges[profile.Pmc._id].Items.New.AddRange(stack);
}

Look into if unifying the code actually makes sense to do so, and update all places where it makes sense to do this