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

Server/Plugins/APIDump/APIDesc.lua

@@ -3317,6 +3317,16 @@ local Hash = cCryptoHash.sha1HexString("DataToHash")
 					},
 					Notes = "Returns the weapon that the entity has equipped. Returns an empty cItem if no weapon equipped or not applicable.",
 				},
+				GetOffHandEquipedItem =
+				{
+					Returns =
+					{
+						{
+							Type = "cItem",
+						},
+					},
+					Notes = "Returns the item that the entity has equipped on off-hand. Returns an empty cItem if no item equipped or not applicable.",
+				},
 				GetGravity =
 				{
 					Returns =