Allow product panel extensions in dashboard

[brucewyne]

when in the dashboard editing a product, the panel on the left should be extendable so that add-ons can save data along with the product.

This will require:
making a class to handle installing product panels. e.g. /VividStore/Product/ProductPanel
making an interface class. e.g. /VividStore/Product/ProductPanellInterface
setting a standard elements path to include the panel content: /elements/product_details/pkg_handle.php