Remove reflection-based DualWielding implementation, and update related utilities

[prokopyl]

This PR does the following:

• Remove the DualWielding reflection-based utility class since all methods are now in Bukkit (as discussed in Remove reflection-based DualWielding implementation #43)
• Move the still useful DualWielding.getHoldingHand() method to InventoryUtils
• Also move the ItemUtils.breakItemInHand() methods to InventoryUtils, as they are more related to inventories than items
• Move the DualWielding enum to InventoryUtils, now as a simple enum representing the hands
• Change ItemUtils.consumeItem() to ItemUtils.consumeItemInMainHand and ItemUtils.consumeItemInOffHand (as discussed in ItemUtils: Update consumeItem to support dual-wielding #41)
• Rename ItemUtils.damageItemInHand() to ItemUtils.damageItem(), and update it to use the Damageable API (see ItemUtils: Update damageItemInHand to use the Damageable ItemMeta API #42)

This fixes #41.
This fixes #42.
This fixes #43.

[AmauryCarrade]

Nice changes! Only thing: missing changelog update. At least:

• added ItemUtils.consumeItemInOffHand;
• moved ItemUtils.consumeItem to ItemUtils.consumeItemInMainHand;
• moved ItemUtils.damageItemInHand to ItemUtils.damageItem;
• ItemUtils.damageItem now returns true if the damaged item was broken;
• moved ItemUtils.breakItemInHand methods to InventoryUtils.breakItemInHand;
• moved DualWielding to InventoryUtils.DualWielding;
• moved DualWieldling methods to InventoryUtils;
• removed DualWieldling.setItemInHand and DualWieldling.getItemInHand (use Bukkit API instead).