Fixes and enhancements

• Fixed problem that would prevent the application of Features on symbols that do not have a Group Code in the Scheme tab.
• Fixed problem with the Scheme builder that could cause duplicate entries of rules when clicking multiple times on Apply to section.
• Fixed problem with Mirror Product that could cause a dropdown entry called <Bad Reference> to show up after removing mirroring from a product.
• Fixed problem with Model previews that could cause the object to show up as black.
• Fixed problem with the Batch Converter that could result in models being converted using the wrong scale.