Moved potion static functions to EntityEffect to create splash potions through the world

Caveat emptor: this *may* break a plugin that was made within the last few days and uses potions

Moved the static functions to `EntityEffect`, and made `SplashPotionEntity` take a `cItem`. This way, we can use `cProjectileEntity::Create` to spawn splash potions instead of the roundabout way that was taken before. The splash potion will generate the three fields that `cItem` replaced in the constructor. 
You're making a copy of the potentially large `cItem` object here, only to "ask it a few questions", can't you use a `const cItem &` instead?