Set up inventory to only allow weapons to be equipped if their ammo count is not 0

[trundlr]

UI:
In the Inventory UI, if a Weapon's Ammo is 0, the InventoryItem panel should be disabled.

Player Inventory:
Perform a check on the Weapon's Ammo before setting the weapon as active that ensures the Ammo is not 0.

[matekdev]

Maybe we should private the Ammo field in the future? For now, this should be fine
5901c1c