Move multiplayer classes to "core/multiplayer" subdir.
Move the RPCConfig and enums (TransferMode, RPCMode) to a separate
file (multiplayer.h), and bind them to the global namespace.
Move the RPC handling code to its own class (RPCManager).
Renames "get_rpc_sender_id" to "get_remote_sender_id".
For the time being we don't support writing a description for those, preferring
having all details in the method's description.
Using self-closing tags saves half the lines, and prevents contributors from
thinking that they should write the argument or return documentation there.
Used by ENetMultiplayerPeer and WebSocketServer to generate network IDs,
and exposed to the user for p2p networks (e.g. WebRTCMultiplayerPeer)
and custom MultiplayerPeer implementations.
Fabio Alessandrelli
Rémi Verschelde