switched back to std::vector for the WeaponSlots to keep them in the same order as in the XML file
added DefaultWeaponmodeLink, a class which links weaponmodes (a property of a Weapon or a WeaponPack) with firemodes (one firemode corresponds to one WeaponSet). This can be changed later (for example in a nice GUI), but DefaultWeaponmodeLink defines the default value.