Allow Inventory on Main Product or Variants

Allow setting inventory on either product/main item, or variants.
Currently, inventory can only be set at the variant level, if variants are enabled.
Scenario 1: You have a t-shirt design offered on 3 colors of shirts. It makes sense for each variant to have an inventory level. ie. 2 white shirts, 3 ivory shirts, 4 gray shirts.
Scenario 2: I am selling a handmade one-of-a-kind pendant with customer's choice of chain length. I only have one pendant so inventory needs to be set at the main product level.
See screen shots.
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