FIXED BUGS WHEN LOADING INTO THE MISSION

- briefing.sqf: missing ','
- fnc_exec_console.sqf: completely disabled addon "proving_ground". Would have needed to be disabled for live deployment anyway since its more geared towards testing and debugging
- onMouseWheel.sqf: constantly tried to "terminate" a variable that has not been initialized yet. Fixed that.
- OnKeyPressed.sqf: was only called at one place and then w/o any arguments which the script itself dislikes. Commented out.