Optimize API/Events for TabList

[Joo200]

API Changes

Currently the changes from the backend servers are passed to the client without any event or api to interrupt/change stuff. E.g. it's not possible to change the display name without hacks. It would be nice to have an event to modify the added or changed tablist item and to prevent the proxy from sending those changes.

Another reason to change stuff there

Currently modifications at the player list will not affect the sent package. This may be a problem: In VelocityTabList#processBackendPacket the key should be replaced by the public key when no key is present from the backend server. I don't know how this should happen but with the current implementation the key of the player will only be replaced on the server, the package is sent without any modification.