Fix a major bug and add some stuff

Fix an error when the timer was cancelled.
Add some methods to get all slots in the border and set an item on them.
Dynamic plugin.yml thanks to gradle.
Set spigot's version to 1.16.5.
Change api-version to 1.16.
Compile to UTF-8.

Change version from 1.0.2 to 1.0.3.

Add an other creation method

An InventoryAPI's object can be created with a class which extends JavaPlugin. An object of this class can be use too.

Fix Error

This release fiw an IllegalStateException

First release

First release of InventoryAPI