Add scenario condition game_started

This should be used to call events without any delays when the game starts. Fixes #1576 
The problem is that I had to set the delay to 0 in show_logbook_entry_delayed(). We'll need to decide whether to keep it this way ( I like it better this way, since I don't have to wait about 3 seconds to get feedback after completing a scenario goal ) or send a parameter to the function specifying the delay.