Revert "Allow element callbacks to be regular plugin methods. Update PriceDefaultWidget."
The used approach doesn't work correctly with widgets or formatters because
they require additional configuration to be passed to the reinstantiated plugin.
This reverts commit 90d9d8e7f34a44d48ae53033e97b7ab0e2524b5a.
Rework the product sku constraint.
- Shorten the error message.
- Revert the per-store validation, it's not feasible the planned product architecture.
- Use the Symfony 2.5+ API for adding violations, instead of the deprecated one.