Redesign of message decoders

[lare96]

They should be able to define an event type to decode, and have the option to handle it additionally once the event object has been created.

The drop, equip, etc. decoders should be renamed to something different, as they don't just serve solely that purpose. I'm currently unaware of the specific trigger that makes the client send a different packet for the same context slot. Will have to take a look at the client to find out