Save ingame-time in commands.txt

[elexis]

This ticket is a preparation for the use of variable turn lengths (#69). In case we use them, but don't update the format of the commands.txt file, we can only derive the ingame time by summarizing all turn lengths of all previous turns.

• Debugging: If you want to find the commands issued in a commands.txt file at a given time, you'd have to run a script to find the looked-for turn number. Even without the variable turn length, you have to compute the turn number first, given the ingame time. If we save the ingame time like "hh:mm:ss.ms" then you can find the given turn immediately.

• Performance boost replay-menu: (#3258) The performance imrpovement of at least 2 orders of magnitude can be kept, as long as only the last line of the commands.txt file needs to be read to find the total duration of the game.