Change to an case insensitive approach for "visibility"

[vadimjustus]

For visibility, there is a bunch of predefined values possible:

• Not Visible Individually
• Catalog
• Search
• Catalog, Search

There are validation logic and business logic for these values in the code, which is case sensitive. In the scope of an improvement it should be evaluated, whether it is possible to make the handling of these values case insensitive.