Added GenericActionPacket - read description

The GenericActionPacket is a packet to replace the PingRequest,
PingResponse and the TerminateWork packets (and any other packet that
just indicates that some action is to be taken on the receiver end for a
packet that doesn't involve a Timer).
I'll do up another Generic Packet type for Packets that include Timers,
and I'll also try make one that can act for all packets bar those that
send data