This Minecraft Plugin Library will help you for custom items in your guis.
This library makes the creation of custom items faster for you. So you can create items and insert them when you are making your gui.
Nope ! I'm currently working on upgrading it as much as I can and maximize the possbilities of this library.
I honestly got tired of making custom items one by one and decided to make custom functions/methods to create them.
When making your Minecraft plugin, simply add the jar of this in the external libraries. Then You can use the library in your plugin
Not already but I plan on trying to do something so it'd be possible. Please note that I will make it MongoDB friendly before MariaDB.