This is a continuation of #70 / #84

Unfortunately, FAPI doesn't give us a lot of options here (we'd have to fuck with internals or add our own registry for this), but basically, I'd like us to be able to retrieve a list of all trades during trade registry (and maybe also after the "initial" registry has fired, in a separate event), so we can allow for further modification like removing or editing existing trades. The reason for this being that there is no way to "override" existing trades with our current system, which is something both modders and packmakers may want to do.

This will definitely require more discussion to shape exactly how this should / shouldn't be done, what things we should give modders access here (and how) and so on, so feel free to contribute your own ideas here, as well!