Move image "widget" settings from Store configuration to Ubercart Product field formatter configuration

[bugfolder]

Follow-on to #245 (comment).

Eventually, it [is] a good idea to move the selection of the image "widget" (which, in fact, it's a formatter, not a widget) to the field formatter configuration in Manage display, when the user chooses the "Ubercart product" formatter. That way, the selection of the actual plugin is done right there rather than having to go to the Store menu.

H/T @argiepiano

[bugfolder]

Currently, the "image widget" setting is at admin/store/settings/products, in the "Product settings" vertical tab.