Fix fixed weapon

Since latest changes in upstream, hard to support merging in addItem() member function.
This commit does the same things as https://github.com/Yankes/OpenXcom/pull/6
But all changes in logic isolated to one separate block. Therefore new changes in upstream will be easy to merge.
```
	// fixed weapon should be always placed in hand slots
	if (item->getRules()->isFixed())
	{
		if (!rightWeapon || !leftWeapon)
		{
			item->moveToOwner(unit);
			item->setSlot(!rightWeapon ? rightHand : leftHand);
			placed = true;
			_save->getItems()->push_back(item);
			item->setXCOMProperty(unit->getFaction() == FACTION_PLAYER);
		}
		return placed;
	}

	bool keep = true;
	switch (item->getRules()->getBattleType())
	{
		blablabla;
	}
```

This commit does the same as https://github.com/SupSuper/OpenXcom/pull/954/files