Start removal of categories.

To-Do:
* Go through all utils, make it neater and remove useless methods. Also
make sure util methods are used where they should be to remove
duplication.
* Remove the extra command classes and make it all just one class.
* Other stuff I'm too sick to think of
Move the commands around.
Made sure InventoryUtil.doesBlockHaveInventory was used where it should
be, it will slightly increase performance.
A core package seems nicer. Less clutter in mechanics.