Treat ammo slots the same way as quickbar

[eterevsky]

Now that quickbar acts as a collection of shortcuts instead of inventory slots, it seems logical that the ammo slots should work the same way.

Pros:
- you will see the actual number of bullets left in your inventory,
- when you are loading a gun turrent from your ammo slot, it will not stay empty.

Cons:
- It will become slightly more difficult to switch between ammunition types for the same weapon.