Core/Cleanup: remove C-style cast to (Bag*):

* add ToBag() methods for Item class;
* add GetBagByPos() method for Player class.

src/server/game/Entities/Item/Item.cpp

@@ -281,6 +281,15 @@ bool Item::Create(uint32 guidlow, uint32 itemid, Player const* owner)
     return true;
 }
 
+// Returns true if Item is a bag AND it is not empty.
+// Returns false if Item is not a bag OR it is an empty bag.
+bool Item::IsNotEmptyBag() const
+{
+    if (const Bag* pBag = ToBag())
+        return !pBag->IsEmpty();
+    return false;
+}
+
 void Item::UpdateDuration(Player* owner, uint32 diff)
 {
     if (!GetUInt32Value(ITEM_FIELD_DURATION))