Adds a "TradeComplete" method to SteamBot

I noticed SteamBot doesn't actually do anything with the Trade Manager's `trade.HasTradeCompletedOk`, so I added an event that fires when `trade.HasTradeCompletedOk` is `true`.

This should theoretically fix the issue of SteamBot not knowing whether the trade went through successfully when there are a large amount of items.

**Someone please test this first.**

If working as intended, you would move your methods that run after a successful trade into `OnTradeComplete()` instead of `OnTradeClose()` or wherever you normally have it.