Add product title/SKU autocomplete for forms to use

This patch adds an autocomplete function for forms to use that need to autocomplete on product titles/SKUs. My hunch is this can be revised in the future to include arguments that people can specify when they set their form's #autocomplete_path. We'll need to develop a few of these for scaling reasons. I'm not sure where this should be used in core, but it can certainly wind up in CA conditions and in the order edit screen.