Improvements to login handling. Fixes #65, #55.

This will possibly resolve issues with identities being switched once
and for all.

1. Server should not start the setup GUI while ticking. It should
instruct the client to open that GUI after creating a world properties
manager for the newly logged in player. That way, the player is
guaranteed to have received the WPM. Blank GUI was caused by a WPM not
being present for the client.

2. ClientTickHandler should reset the hasCompletedMainMenuTick field
when the in-game menu is hit. This will cause the world manager map to
clear properly when exiting a single player game or a server game.

3. When a connection is closed, the server will unload that player's
world properties manager from its memory.

4. The server will no longer pre-load all world properties managers
present in the config folder. They will be loaded as-needed as each
player logs in.