Added item combines with equipped stack instead of creating a new unequipped stack

The current OpenMW behaviour does not imitate vanilla behaviour.

Steps:

Equip a stack of ammo. Iron Bolt, for example.

In the console, while inventory is still open, do 'additem "iron bolt" anycount'

Close the console and you can see that the newly added item combines with the equipped stack,

In vanilla, this is not the case.

Additionally, it seems vanilla updates the stack count visual even in menumode. You can see that the items were added without needing to close the console.

At first glance, a solution for this issue would be to prevent auto-equipping of ammo if the same type of ammo is already equipped.

Somewhat related to #4754 (closed), but just close this issue if you think it's a duplicate.