Avoid direct usage of it_container::contains

Use item::get_remaining_capacity_for_liquid instead.
Implemented the item slot for container items:

This slot stores container content size (not item size).
Implemented the item slot for armor items:
Add armor slot to some bows to make them wearable.
Add armor slot data to wearable containers.

Removes the IS_ARMOR tag because the presence of the armor slot data makes the item armor.