Migrate speed, damage/armor and firepower modifiers to integer

Current state:
SpeedModifier seems to work fine.

DamageModifier currently does about twice as much damage as it should, probably a misplaced division op, haven't found the error yet.

FirepowerModifier leads to zero damage of tanks vs. infantry for example, so there's probably a misplaced division op as well.
Remove the `int` cast here please.