Implemented item pickup/dropping/equipping. ...

Also expanded the test world and some other adjustments.

Some refactoring and bug fixing (introduced debugging print function).

Added single-player mode, enabled saving/loading of a game. ...

Also related code cleanup.

First commit on the single-player branch. ...

Networking is proving to be a major challenge, so I'm bypassing it for now...
Refactored the start menu.