Fixed loading structure modifiers from ini-file
Implement periodical damage instead of incendiary damage

1. Renamed all incendiary damage stuff, including field names in ini-files
proj_checkBurnDamage(.. -> proj_checkPeriodicalDamage(..
incenTime -> periodicalDamageTime
incenDamage -> periodicalDamage
incenRadius -> periodicalDamageRadius
incenWeaponClass -> periodicalDamageWeaponClass
incenWeaponSubClass -> periodicalDamageWeaponSubClass
incenWeaponEffect -> periodicalDamageWeaponEffect
burnStart -> periodicalDamageStart
burnDamage -> periodicalDamage
//change comments containing 'incen' and 'burn'

2. Add damage classes for periodical damage
Added 3 parameters loaded from weapons.ini:
periodicalDamageWeaponClass
periodicalDamageWeaponSubClass
periodicalDamageWeaponEffect

Propulsion and body types now taken into account when periodical damage calculated.