Standard product not in "Add..." product admin menu

[mwort]

Since the custom product models were introduced in 8d3ac6b the "Add product" button in the Products admin has been replaced by a "Add..." menu, but the standard Product model doesnt appear as an addable item. I.e. there is no way to add a standard product in the products list admin and basically means a custom product has to be created, which I dont think is intended.
Custom Product models that have been subclassed from the standard Product do show up though.

I have tested this with the latest master of mezzanine and cartridge, but havent found a non-hacky way to add it.