Fix item removed event

Fix for the issue of Steam not correctly publishing the ItemAdded/Removed events sometimes.  See https://github.com/Jessecar96/SteamBot/issues/602

Note that there is a small breaking change - I changed the type of `MyOfferedItems` and `OtherOfferedItems` from `List<ulong>` to `IEnumerable<TradeUserAssets>`.  
One quick-fix would be to replace any instance of `Trade.MyOfferedItems` in your UserHandler with `Trade.MyOfferedItems.Select(o => o.assetid)`