Add a Slot#isSameInventory() method (#2884)

This fixes shift + double clicking items in Containers that use
SlotItemHandler slots.

src/main/java/net/minecraftforge/items/SlotItemHandler.java

@@ -77,4 +77,10 @@ public class SlotItemHandler extends Slot
     {
         return itemHandler;
     }
+
+    @Override
+    public boolean isSameInventory(Slot other)
+    {
+        return other instanceof SlotItemHandler && ((SlotItemHandler) other).getItemHandler() == this.itemHandler;
+    }
 }