AdvancedServerList

This is a small update making some improvements to the code, while also fixing a few minor things.

Improvements

• Created a BukkitCore<F> class to extend the JavaPlugin class and implement the PluginCore<F> interface respectively. This allowed me to simply provide the class instance for cases where either a JavaPlugin or a PluginCore instance was needed.
• Made AdvancedServerList have a Type parameter (So it became AdvancedServerList<F> This was made to get rid of PluginCore<?> usage, which I wasn't a big fan of.
• Unregister PlaceholderAPI expansion when plugin is disabling itself. This should allow support for using /reload (Although I do not recommend using that).

Fixes

• PlaceholderAPI expansion not being set as Persistent. It should now no longer be unregistered when using /papi reload
• JoinEvent class not being registered as a Event listener