Added a fix for the double chest Issue #2493
The data wasn't always going into the doublechest, and didn't appear to be combining correctly upon logout/login of server.
The doublechest, now starts with the items created from left/right in it's constructor.
Then in it's setContent's function. It now doesn't call parent (as the slot check is invalid and won't check both chests) but now has a similar pattern using the slots of left and right.
When you set contents to an inventory, I now clear if the 'SetItem' fails.
Added double chest fix
Could you follow the Contribution Guidelines? Specifically the code formatting, so it follows the project's formatting
Implemented double chest fix, closes #2744, fixes #2493