Fixed hotbar swapping glitch:
I experimented with detecting the hotbar swap, but canceling one of those in a plugin is pretty buggy. While it didn't actually dupe or delete any shulker boxes, it looked like it did until they clicked on the empty slot or copy of the shulker box and the box re-appeared or the copy disappeared. I figured that was kind of confusing, so I just added a check when the ender chest is closed (or opened) that automatically moves all shulker boxes to their inventory.

Basically, it will look like they're able to use the hotbar swap to put shulker boxes in until they close the chest and the boxes are back in their inventory.