InfantryWeaponMounted was not a subclass of WeaponMounted but of Mounted instead. I assume this was done to keep the IWM class simple and not influenced by WeaponMounted stuff that it would never need. Unfortunately this meant that infantry with secondary weapons got no weapon attacks, as IWMs were not entered into a unit's weapon list (and could not without bigger changes - List). So, I made IWM a subclass of WeaponMounted and in nothing short of a miracle, it immediately compiled without any error and I could not find any errors when I set up a game where I shot at a target mek with two infantry units. We may of course find some error somewhere later because of this but I feel that mangling Entity's weaponList would not be the right way to go.

Fixes #5506