Support swap item between main hand and offhand by press key "F" (#4241)

Hi,

I found cuberite don't support to press key "F" to swap items on main hand and offhand, so I implemented this feature.

Best regards
Changyong

src/Entities/Entity.h

@@ -351,6 +351,9 @@ public:
 	/** Returns the currently equipped boots; empty item if none */
 	virtual cItem GetEquippedBoots(void) const { return cItem(); }
 
+	/** Returns the currently offhand equipped item; empty item if none */
+	virtual cItem GetOffHandEquipedItem(void) const { return cItem(); }
+
 	/** Applies damage to the armor after the armor blocked the given amount */
 	virtual void ApplyArmorDamage(int DamageBlocked);
 

src/Entities/Player.h

@@ -71,6 +71,9 @@ public:
 	/** Returns the currently equipped boots; empty item if none */
 	virtual cItem GetEquippedBoots(void) const override { return m_Inventory.GetEquippedBoots(); }
 
+	/** Returns the currently offhand equipped item; empty item if none */
+	virtual cItem GetOffHandEquipedItem(void) const override { return m_Inventory.GetShieldSlot(); }
+
 	virtual void ApplyArmorDamage(int DamageBlocked) override;
 
 	// tolua_begin