Moves my (sabirm)'s custom item to common loadout pool

Moves the white sundress to the common pool. Same cost as the regular sundress, 3 points. Also adds a sprite for it when it's on the ground/in the inventory.

As requested (by myself). I'm not sure how the other changes in item_defines.dm were made, but it doesn't look like it has actually changed anything. Just deleted some spaces automatically, I suppose.

You can comment out the common loadout addition, if you'd like, and will need to modify the parts I can't access.