Refactor shell movement, modularised in a tweakable way

src/Dodge/Default/Weapon.hs

@@ -37,3 +37,9 @@ defaultGun = Weapon
     , _itTargeting = Nothing
     , _itWorldTrigger = Nothing
     }
+defaultAutoGun :: Item
+defaultAutoGun = defaultGun 
+    { _itScrollUp   = const id
+    , _itScrollDown = const id
+    , _itInvDisplay = basicWeaponDisplay
+    }