Add game_started condition.

This is used to call events when the game starts, without any delay.
Change tutorial show_logbook condition to game_started, to avoit delays.
Check for condition 'game_started' when scenario event handler starts.
Start scenario event handler when session starts.
Remove print statement used for debugging.
Rewrite game_started condition to make it nicer.
Change The_Unknown_en.yaml to use game_started condition
Merge pull request #2100 from unknown-horizons/scenario_conditions

Add scenario condition game_started