Add messages to message area when a player joins or leaves the game

Add messages to message area when a player joins or leaves the game

This replaces code we had previously for adding a message to the
message area when we completed sending a register request. The new
code is more reliable in the sense that a message is added _only_
after the server reports the event for a new player. (Previously, if
the server was offline, a message would still appear saying "Player is
now in the game!" which would be false and confusing if there was no
server responding to the register request at all.)