add CBA_missionTime

[commy2]

BI seems to be unable to make a proper time command that is synched in MP and is correctly resumed in save games, JIP and all that.

This PR adds the CBA_missionTime variable.

pros:

• based on tickTime
• set every frame
• synched in MP/JIP
• resumes correct value when loading save games
• multiplied by accTime (SP only)
• paused when game is paused (ESC menu, SP only)

cons:

• -1 on clients during preInit (correct value on and after postInit)
• not linked to ingame day time (faster)
• ignores skipTime, setDate, timeMultiplier etc. (ingame day time)

Test:
https://gist.github.com/commy2/c01e4885b8961363ef7e
https://gist.github.com/commy2/06de40913799965ba8e5

0 = 0 spawn {waitUntil {diag_log CBA_missionTime; systemChat str CBA_missionTime; false}};

I ran it for 2hrs and the values didn't diverge. (Although it was two game instances on the same machine)

[nicolasbadano]

Looks good to me. This is critical to support things that require timing across save/load games.