ItemData-Updates

We added a new empty constructor to the ItemData-class. We also added
ItemStack-Block-Comparation utility methods to the ItemData-class.
InventoryUtil Clean

We cleaned up the InventoryUtil- and the ItemData-class and changed some
JavaDoc in the Inventory Utility.
Updater Code Fix

The tryInstall()-method in the updater without a command executor now
also returns success.