added a "world thread" which automatically stops when World.clean() has been called

This thread does nothing yet, except sending an INFO level debug message every second. The goal is to prevent the world simulation from slowing the UI.
now commands are stored in the world queue before being executed

Note: the game might have a strange behavior because the events are still rendered from the world thread!
removed safe_update()
bug fixed: impossible to save a game

Now the command queue is excluded from the pickle. Thanks to Christopher Toth for reporting the bug.
added "Making AIs Tutorial"

thanks to Thiago Seus
if the game is starting, the "no unit defined" warning won't be issued
Merge branch 'fast-ui'
bug fixed: the full screen mode wouldn't work if the video card maximum resolution is too big for the monitor