prevent dropping of undropable items

As dwarfkoala suggested, there is a flag.

The advanced inventory should not be affected as it has no access to the weapon slot. Wielding something else from the adv. inv. fails with the appropriate message. Of course if the item "somehow" appears in the normal inventory, the adv. inv. will drop the item. But the actually problem is than how the item got there in the first place.

I also added the hint to the item menu that makes the 'd' for dropping gray instead of green for these kind of items.

Fixes #6187