* Fixed dupe bug
* Changed the way items store*
+ Updater

After a critical dupe bug** found by @matahombres,I implemented a fix and refacored most of the code. I also added an updater so you can get notified when an update is released (if you have permission bettershulkerboxes.updatenotify). When possible, it will check latest version avaible at spigot and compare to the one you're using.

* = From version 1.2 items started to be stored with each inventory click to prevent duping on death and when dropping a shulkerbox, this caused the bug. Now it just saves when you close the inventory, your HP reaches 0. It also prevents you from dropping the shulker box.
** = If a selected shulker box was double clicked inside an inventory (ex: chest) it would copy the items from that chest.

Image: @matahombres