Work around a 1.13 bug that causes 1.13 inventory holders not to work anymore.
While testing this plugin, I encountered this bug:
https://hub.spigotmc.org/jira/browse/SPIGOT-4274.
This update works around the issue and prepares for the future bukkit updates where custom inventory holders
will likely be deprecated. Sadly, this update is a small breaking change for plugins that shade GuiLib. The GuiListener now uses the singleton pattern so that GuiInventoryHolders without a specified listener default to the single instance listener.