Locked slots can be emptied

Locking a slot should ensure that there's always 1 item left in that slot.

But you can:

• shift click all items out of a locked slot, leaving it empty (see also Tfarcenim/Dank-Storage#108)
• pull all items out of a docked dank using a hopper
• use up the last item in construction mode

Slots unlocking themselves

There's also the issue that locked slots can unlock themselves by:

• placing a Dank in a Dock
• entering construction mode and changing the current slot (shift-scroll)

I'm not sure what the exact conditions are, but slots can randomly unlock themselves as well and even worse:
Locking does sometimes not actually lock the slot; it shows the slot is red, but opening the dank again shows it is unlocked.

When a slot is locked, you see the dank moving up in your hand as if you freshly picked it up. This is because the NBT has changed.
When slot locking doesn't work, this animation is missing, suggesting it doesn't set the NBT.

Steps to reproduce

• Give yourself a fresh dank
• Open it, notice the animation because NBT is added the first time
• Place an item inside, this updates NBT
• Lock the slot, this updates NBT
• Close the dank

If you look at the NBT of the item, it should correctly have the item inside and the slot locked.

• Open it again, this updates the NBT again(!) and the slot appears to be locked, but in reality it is already unlocked

At this point, trying to lock slots doesn't update NBT, but adding items does, which can be used to force slots to lock.

Problem was lack of nbt saving, fixed.