Refactor shell movement, modularised in a tweakable way

src/Dodge/Default/Shell.hs

@@ -8,10 +8,12 @@ defaultShell = Shell
     { _pjPos = (0,0)
     , _pjStartPos = (0,0)
     , _pjVel = (0,0)
+    , _pjAcc = (0,0)
     , _pjDir = 0
     , _pjSpin = 0
     , _pjDraw = \_ -> blank
     , _pjID = 0
+    , _pjTimer = 0
     , _pjUpdate = \_ -> id
     , _pjPayload = \_ -> id
     }

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
+    }