Adds User Handler feedback on bot's trade request accept/decline.

A few users have asked for this before, and since I needed it myself, here you go.

Adds a new user handler event `OnTradeRequestReply()`. It's not critical to add it, your code will compile just fine without it.

The event will return `true` if the trade request sent by the bot has been accepted, `false` otherwise.
It will also provide you with a `string` of the callback response message.