Save game compatible code

1. New Global Variables.

You can add any new global variable in the module system. Compiler will add a record in the file variables.txt. Don't delete this file from module folder. It will be generated with new order.

2. Simple Triggers.

You can add new simple triggers. But you can't delete just place en empty trigger. Total count should be the same.

3. Inserted new items, new factions, new troops, etc. esp. that change constants like mercenaries_end, books_begin, etc

You can create placeholders for new records or add them in the end.